feat: add new user-agent-metadata flag (#2302)

## Description

Add a new `--user-agent-metadata` flag that allows user to append
additional user agent metadata. The flag takes in []string and will
concatenate it with `.`.

```
go run . --user-agent-metadata=foo
```
 produces `0.25.0+dev.darwin.arm64+foo` user agent string

```
go run . --user-agent-metadata=foo,bar
```
produces `0.25.0+dev.darwin.arm64+foo+bar` user agent string

## 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:

- [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/genai-toolbox/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 #<issue_number_goes_here>

cmd/root.go

@@ -386,6 +386,7 @@ func NewCommand(opts ...Option) *Command {
 	// TODO: Insecure by default. Might consider updating this for v1.0.0
 	flags.StringSliceVar(&cmd.cfg.AllowedOrigins, "allowed-origins", []string{"*"}, "Specifies a list of origins permitted to access this server. Defaults to '*'.")
 	flags.StringSliceVar(&cmd.cfg.AllowedHosts, "allowed-hosts", []string{"*"}, "Specifies a list of hosts permitted to access this server. Defaults to '*'.")
+	flags.StringSliceVar(&cmd.cfg.UserAgentMetadata, "user-agent-metadata", []string{}, "Appends additional metadata to the User-Agent.")
 
 	// wrap RunE command so that we have access to original Command object
 	cmd.RunE = func(*cobra.Command, []string) error { return run(cmd) }

cmd/root_test.go

@@ -70,6 +70,9 @@ func withDefaults(c server.ServerConfig) server.ServerConfig {
 	if c.AllowedHosts == nil {
 		c.AllowedHosts = []string{"*"}
 	}
+	if c.UserAgentMetadata == nil {
+		c.UserAgentMetadata = []string{}
+	}
 	return c
 }
 
@@ -230,6 +233,13 @@ func TestServerConfigFlags(t *testing.T) {
 				AllowedHosts: []string{"http://foo.com", "http://bar.com"},
 			}),
 		},
+		{
+			desc: "user agent metadata",
+			args: []string{"--user-agent-metadata", "foo,bar"},
+			want: withDefaults(server.ServerConfig{
+				UserAgentMetadata: []string{"foo", "bar"},
+			}),
+		},
 	}
 	for _, tc := range tcs {
 		t.Run(tc.desc, func(t *testing.T) {

docs/en/reference/cli.md

@@ -27,6 +27,7 @@ description: >
 |              | `--ui`                     | Launches the Toolbox UI web server.                                                                                                                                              |             |
 |              | `--allowed-origins`        | Specifies a list of origins permitted to access this server for CORs access.                                                                                                     | `*`         |
 |              | `--allowed-hosts`          | Specifies a list of hosts permitted to access this server to prevent DNS rebinding attacks.                                                                                      | `*`         |
+|              | `--user-agent-extra`       | Appends additional metadata to the User-Agent.                                                                                                                                   |             |
 | `-v`         | `--version`                | version for toolbox                                                                                                                                                              |             |
 
 ## Examples

internal/server/config.go

@@ -64,12 +64,14 @@ type ServerConfig struct {
 	Stdio bool
 	// DisableReload indicates if the user has disabled dynamic reloading for Toolbox.
 	DisableReload bool
-	// UI indicates if Toolbox UI endpoints (/ui) are available
+	// UI indicates if Toolbox UI endpoints (/ui) are available.
 	UI bool
 	// Specifies a list of origins permitted to access this server.
 	AllowedOrigins []string
-	// Specifies a list of hosts permitted to access this server
+	// Specifies a list of hosts permitted to access this server.
 	AllowedHosts []string
+	// UserAgentMetadata specifies additional metadata to append to the User-Agent string.
+	UserAgentMetadata []string
 }
 
 type logFormat string

internal/server/server.go

@@ -64,7 +64,11 @@ func InitializeConfigs(ctx context.Context, cfg ServerConfig) (
 	map[string]prompts.Promptset,
 	error,
 ) {
-	ctx = util.WithUserAgent(ctx, cfg.Version)
+	metadataStr := cfg.Version
+	if len(cfg.UserAgentMetadata) > 0 {
+		metadataStr += "+" + strings.Join(cfg.UserAgentMetadata, "+")
+	}
+	ctx = util.WithUserAgent(ctx, metadataStr)
 	instrumentation, err := util.InstrumentationFromContext(ctx)
 	if err != nil {
 		panic(err)