feat(storage): add OpenTelemetry tracing for OAuth2 token source

[sahusneha2004]

Description

This PR introduces OpenTelemetry tracing support for authentication token refreshes (Auth.RefreshAccessToken) to fulfill the T5 internal operations telemetry specification for the Google Cloud Storage Go client.

Background & Architectural Context

In the Go Cloud SDKs, default credential resolution (Application Default Credentials) and background token lifecycle management are handled deep within the shared transport layer (google.golang.org/api/transport and cloud.google.com/go/auth).

To provide Auth.RefreshAccessToken span recording within the storage module without modifying upstream shared transport dependencies, this PR introduces TracedTokenSource as a non-intrusive decorator around oauth2.TokenSource.

Includes unit test TestTracedTokenSourceSpan in storage/trace_test.go.

Usage Example

Callers or integrations supplying a custom oauth2.TokenSource can wrap it with NewTracedTokenSource:

ts := storage.NewTracedTokenSource(customTokenSource)
client, err := storage.NewClient(ctx, option.WithTokenSource(ts))

[gemini-code-assist]

Code Review

This pull request introduces TracedTokenSource, which wraps an oauth2.TokenSource to record OpenTelemetry spans for authentication token refreshes, along with corresponding unit tests. The feedback highlights a critical compilation error in Token() where a context.Context is incorrectly passed to endSpan instead of a trace.Span, and notes that the new TracedTokenSource is not yet integrated into the client initialization path.