feat(tool/looker): add support for "description" field in looker tool (#1199)

## Description
---
Adds support for fetching description for looker dimensions and
measures.
Descriptions give important context to any agent about how to filter and
aggregate.

Adds the `description` DimensionFields and MeasureFields
Handles cases when description is not available.
Adds tests.

## PR Checklist
---
- [X] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [X] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/langchain-google-alloydb-pg-python/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [X] Ensure the tests and linter pass
- [X] Code coverage does not decrease (if any source code was changed)
- [X] Appropriate docs were updated (if necessary)
- [X] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #1197

---------

Co-authored-by: Magnus Benediktsson <magnus.benediktsson@nordnet.se>

internal/tools/looker/lookercommon/lookercommon.go

@@ -24,15 +24,20 @@ import (
 )
 
 const (
-	DimensionsFields = "fields(dimensions(name,type,label,label_short))"
-	FiltersFields    = "fields(filters(name,type,label,label_short))"
-	MeasuresFields   = "fields(measures(name,type,label,label_short))"
-	ParametersFields = "fields(parameters(name,type,label,label_short))"
+	DimensionsFields = "fields(dimensions(name,type,label,label_short,description))"
+	FiltersFields    = "fields(filters(name,type,label,label_short,description))"
+	MeasuresFields   = "fields(measures(name,type,label,label_short,description))"
+	ParametersFields = "fields(parameters(name,type,label,label_short,description))"
 )
 
 // ExtractLookerFieldProperties extracts common properties from Looker field objects.
 func ExtractLookerFieldProperties(ctx context.Context, fields *[]v4.LookmlModelExploreField) ([]any, error) {
-	var data []any
+	data := make([]any, 0)
+
+	// Handle nil fields pointer
+	if fields == nil {
+		return data, nil
+	}
 
 	logger, err := util.LoggerFromContext(ctx)
 	if err != nil {
@@ -56,6 +61,9 @@ func ExtractLookerFieldProperties(ctx context.Context, fields *[]v4.LookmlModelE
 		if v.LabelShort != nil {
 			vMap["label_short"] = *v.LabelShort
 		}
+		if v.Description != nil {
+			vMap["description"] = *v.Description
+		}
 		logger.DebugContext(ctx, "Converted to %v\n", vMap)
 		data = append(data, vMap)
 	}

internal/tools/looker/lookercommon/lookercommon_test.go

@@ -0,0 +1,162 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package lookercommon_test
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/googleapis/genai-toolbox/internal/testutils"
+	"github.com/googleapis/genai-toolbox/internal/tools/looker/lookercommon"
+	v4 "github.com/looker-open-source/sdk-codegen/go/sdk/v4"
+)
+
+func TestExtractLookerFieldProperties(t *testing.T) {
+	ctx, err := testutils.ContextWithNewLogger()
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+
+	// Helper function to create string pointers
+	stringPtr := func(s string) *string { return &s }
+
+	tcs := []struct {
+		desc   string
+		fields []v4.LookmlModelExploreField
+		want   []any
+	}{
+		{
+			desc: "field with all properties including description",
+			fields: []v4.LookmlModelExploreField{
+				{
+					Name:        stringPtr("dimension_name"),
+					Type:        stringPtr("string"),
+					Label:       stringPtr("Dimension Label"),
+					LabelShort:  stringPtr("Dim Label"),
+					Description: stringPtr("This is a dimension description"),
+				},
+			},
+			want: []any{
+				map[string]any{
+					"name":        "dimension_name",
+					"type":        "string",
+					"label":       "Dimension Label",
+					"label_short": "Dim Label",
+					"description": "This is a dimension description",
+				},
+			},
+		},
+		{
+			desc: "field with missing description",
+			fields: []v4.LookmlModelExploreField{
+				{
+					Name:       stringPtr("dimension_name"),
+					Type:       stringPtr("string"),
+					Label:      stringPtr("Dimension Label"),
+					LabelShort: stringPtr("Dim Label"),
+					// Description is nil
+				},
+			},
+			want: []any{
+				map[string]any{
+					"name":        "dimension_name",
+					"type":        "string",
+					"label":       "Dimension Label",
+					"label_short": "Dim Label",
+					// description should not be present in the map
+				},
+			},
+		},
+		{
+			desc: "field with only required fields",
+			fields: []v4.LookmlModelExploreField{
+				{
+					Name: stringPtr("simple_dimension"),
+					Type: stringPtr("number"),
+				},
+			},
+			want: []any{
+				map[string]any{
+					"name": "simple_dimension",
+					"type": "number",
+				},
+			},
+		},
+		{
+			desc:   "empty fields list",
+			fields: []v4.LookmlModelExploreField{},
+			want:   []any{},
+		},
+		{
+			desc: "multiple fields with mixed properties",
+			fields: []v4.LookmlModelExploreField{
+				{
+					Name:        stringPtr("dim1"),
+					Type:        stringPtr("string"),
+					Label:       stringPtr("First Dimension"),
+					Description: stringPtr("First dimension description"),
+				},
+				{
+					Name:       stringPtr("dim2"),
+					Type:       stringPtr("number"),
+					LabelShort: stringPtr("Dim2"),
+				},
+			},
+			want: []any{
+				map[string]any{
+					"name":        "dim1",
+					"type":        "string",
+					"label":       "First Dimension",
+					"description": "First dimension description",
+				},
+				map[string]any{
+					"name":        "dim2",
+					"type":        "number",
+					"label_short": "Dim2",
+				},
+			},
+		},
+	}
+
+	for _, tc := range tcs {
+		t.Run(tc.desc, func(t *testing.T) {
+			got, err := lookercommon.ExtractLookerFieldProperties(ctx, &tc.fields)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if diff := cmp.Diff(tc.want, got); diff != "" {
+				t.Fatalf("incorrect result: diff %v", diff)
+			}
+		})
+	}
+}
+
+func TestExtractLookerFieldPropertiesWithNilFields(t *testing.T) {
+	ctx, err := testutils.ContextWithNewLogger()
+	if err != nil {
+		t.Fatalf("unexpected error: %s", err)
+	}
+
+	got, err := lookercommon.ExtractLookerFieldProperties(ctx, nil)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	want := []any{}
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Fatalf("incorrect result: diff %v", diff)
+	}
+}

tests/looker/looker_integration_test.go

@@ -624,16 +624,16 @@ func TestLooker(t *testing.T) {
 	wantResult = "{\"group_label\":\"System Activity\",\"label\":\"Content Usage\",\"name\":\"content_usage\"}"
 	tests.RunToolInvokeParametersTest(t, "get_explores", []byte(`{"model": "system__activity"}`), wantResult)
 
-	wantResult = "{\"label\":\"Content Usage API Count\",\"label_short\":\"API Count\",\"name\":\"content_usage.api_count\",\"type\":\"number\"}"
+	wantResult = "{\"description\":\"Number of times this content has been viewed via the Looker API\",\"label\":\"Content Usage API Count\",\"label_short\":\"API Count\",\"name\":\"content_usage.api_count\",\"type\":\"number\"}"
 	tests.RunToolInvokeParametersTest(t, "get_dimensions", []byte(`{"model": "system__activity", "explore": "content_usage"}`), wantResult)
 
-	wantResult = "{\"label\":\"Content Usage API Total\",\"label_short\":\"API Total\",\"name\":\"content_usage.api_total\",\"type\":\"sum\"}"
+	wantResult = "{\"description\":\"The total number of views via the Looker API\",\"label\":\"Content Usage API Total\",\"label_short\":\"API Total\",\"name\":\"content_usage.api_total\",\"type\":\"sum\"}"
 	tests.RunToolInvokeParametersTest(t, "get_measures", []byte(`{"model": "system__activity", "explore": "content_usage"}`), wantResult)
 
-	wantResult = "null"
+	wantResult = "[]"
 	tests.RunToolInvokeParametersTest(t, "get_filters", []byte(`{"model": "system__activity", "explore": "content_usage"}`), wantResult)
 
-	wantResult = "null"
+	wantResult = "[]"
 	tests.RunToolInvokeParametersTest(t, "get_parameters", []byte(`{"model": "system__activity", "explore": "content_usage"}`), wantResult)
 
 	wantResult = "{\"look.count\":"