feat(sources/postgres): add application_name (#1504)

## Description

---
> Should include a concise description of the changes (bug or feature),
it's
> impact, along with a summary of the solution

## PR Checklist

---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [ ] Make sure you reviewed

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

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

🛠️ Fixes #<issue_number_goes_here>

internal/sources/alloydbpg/alloydb_pg.go

@@ -117,11 +117,15 @@ func getOpts(ipType, userAgent string, useIAM bool) ([]alloydbconn.Option, error
 }
 
 func getConnectionConfig(ctx context.Context, user, pass, dbname string) (string, bool, error) {
+	userAgent, err := util.UserAgentFromContext(ctx)
+	if err != nil {
+		userAgent = "genai-toolbox"
+	}
 	useIAM := true
 
 	// If username and password both provided, use password authentication
 	if user != "" && pass != "" {
-		dsn := fmt.Sprintf("user=%s password=%s dbname=%s sslmode=disable", user, pass, dbname)
+		dsn := fmt.Sprintf("user=%s password=%s dbname=%s sslmode=disable application_name=%s", user, pass, dbname, userAgent)
 		useIAM = false
 		return dsn, useIAM, nil
 	}
@@ -141,7 +145,7 @@ func getConnectionConfig(ctx context.Context, user, pass, dbname string) (string
 	}
 
 	// Construct IAM connection string with username
-	dsn := fmt.Sprintf("user=%s dbname=%s sslmode=disable", user, dbname)
+	dsn := fmt.Sprintf("user=%s dbname=%s sslmode=disable application_name=%s", user, dbname, userAgent)
 	return dsn, useIAM, nil
 }
 

internal/sources/cassandra/cassandra_test.go

@@ -36,7 +36,7 @@ func TestParseFromYamlCassandra(t *testing.T) {
 			sources:
 				my-cassandra-instance:
 					kind: cassandra
-					hosts: 
+					hosts:
 						- "my-host1"
 						- "my-host2"
 			`,
@@ -62,7 +62,7 @@ func TestParseFromYamlCassandra(t *testing.T) {
 			sources:
 				my-cassandra-instance:
 					kind: cassandra
-					hosts: 
+					hosts:
 						- "my-host1"
 						- "my-host2"
 					username: "user"
@@ -121,11 +121,11 @@ func TestFailParseFromYaml(t *testing.T) {
 			sources:
 				my-cassandra-instance:
 					kind: cassandra
-					host: 
+					hosts:
 						- "my-host"
 					foo: bar
 			`,
-			err: "unable to parse source \"my-cassandra-instance\" as \"cassandra\": [1:1] unknown field \"foo\"\n>  1 | foo: bar\n       ^\n   2 | host:\n   3 | - my-host\n   4 | kind: cassandra",
+			err: "unable to parse source \"my-cassandra-instance\" as \"cassandra\": [1:1] unknown field \"foo\"\n>  1 | foo: bar\n       ^\n   2 | hosts:\n   3 | - my-host\n   4 | kind: cassandra",
 		},
 		{
 			desc: "missing required field",

internal/sources/cloudsqlpg/cloud_sql_pg.go

@@ -98,11 +98,15 @@ func (s *Source) PostgresPool() *pgxpool.Pool {
 }
 
 func getConnectionConfig(ctx context.Context, user, pass, dbname string) (string, bool, error) {
+	userAgent, err := util.UserAgentFromContext(ctx)
+	if err != nil {
+		userAgent = "genai-toolbox"
+	}
 	useIAM := true
 
 	// If username and password both provided, use password authentication
 	if user != "" && pass != "" {
-		dsn := fmt.Sprintf("user=%s password=%s dbname=%s sslmode=disable", user, pass, dbname)
+		dsn := fmt.Sprintf("user=%s password=%s dbname=%s sslmode=disable application_name=%s", user, pass, dbname, userAgent)
 		useIAM = false
 		return dsn, useIAM, nil
 	}
@@ -122,7 +126,7 @@ func getConnectionConfig(ctx context.Context, user, pass, dbname string) (string
 	}
 
 	// Construct IAM connection string with username
-	dsn := fmt.Sprintf("user=%s dbname=%s sslmode=disable", user, dbname)
+	dsn := fmt.Sprintf("user=%s dbname=%s sslmode=disable application_name=%s", user, dbname, userAgent)
 	return dsn, useIAM, nil
 }
 

internal/sources/postgres/postgres.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/goccy/go-yaml"
 	"github.com/googleapis/genai-toolbox/internal/sources"
+	"github.com/googleapis/genai-toolbox/internal/util"
 	"github.com/jackc/pgx/v5/pgxpool"
 	"go.opentelemetry.io/otel/trace"
 )
@@ -99,6 +100,17 @@ func initPostgresConnectionPool(ctx context.Context, tracer trace.Tracer, name,
 	//nolint:all // Reassigned ctx
 	ctx, span := sources.InitConnectionSpan(ctx, tracer, SourceKind, name)
 	defer span.End()
+	userAgent, err := util.UserAgentFromContext(ctx)
+	if err != nil {
+		userAgent = "genai-toolbox"
+	}
+	if queryParams == nil {
+		// Initialize the map before using it
+		queryParams = make(map[string]string)
+	}
+	if _, ok := queryParams["application_name"]; !ok {
+		queryParams["application_name"] = userAgent
+	}
 
 	// urlExample := "postgres:dd//username:password@localhost:5432/database_name"
 	url := &url.URL{