feat(auth): Implement Actionable Errors logging for HTTP requests (#14265)

This PR introduces structured, OTel-compliant telemetry logging for
actionable errors within the `google-cloud-go/auth` HTTP transport
wrapper (`otelAttributeTransport`).

 Key implementations include:
* **Safe Response Cloning:** Adds a `cloneResponse` mechanism that
buffers up to 1MB of the HTTP response body to parse error payloads
(like quota limits or permission errors) and uses
`io.MultiReader` to transparently stitch the stream back together. This
ensures the downstream generated client can still read the
`http.Response.Body` without hitting stream exhaustion or
 OOM errors.
* **Inline Transport Logging:** Introduces `readResponseError` to
extract semantic attributes (e.g., `error.type`, `ErrorInfo` metadata)
from the cloned response. The structured log is
emitted inline via `slog` directly within the `RoundTrip`'s `onError`
hook, ensuring it properly correlates with low-level T4 network spans.
* **Feature Gated:** The entire logging execution is strictly gated
behind the `gax.IsFeatureEnabled("LOGGING")` experimental flag to
guarantee zero allocations when disabled.
* **Testing:** Updates `httptransport_otel_test.go` to validate error
mappings, OTel semantic conventions, and structured JSON payload formats
with high coverage.

Author: westarle

auth/go.mod

@@ -15,6 +15,7 @@ require (
 	go.opentelemetry.io/otel/trace v1.42.0
 	golang.org/x/net v0.52.0
 	golang.org/x/time v0.15.0
+	google.golang.org/api v0.272.0
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260401001100-f93e5f3e9f0f
 	google.golang.org/grpc v1.79.3
 	google.golang.org/protobuf v1.36.11
@@ -33,5 +34,4 @@ require (
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
-	google.golang.org/api v0.272.0 // indirect
 )

auth/grpctransport/grpctransport.go

@@ -471,14 +471,19 @@ func addOpenTelemetryStatsHandler(dialOpts []grpc.DialOption, opts *Options, end
 	}
 	var staticAttrs []attribute.KeyValue
 	var scopedLogger *slog.Logger
+
+	if gax.IsFeatureEnabled("LOGGING") && opts.Logger != nil {
+		scopedLogger = opts.Logger
+	}
+
 	if opts.InternalOptions != nil {
 		staticAttrs = transport.StaticTelemetryAttributes(opts.InternalOptions.TelemetryAttributes)
-		if gax.IsFeatureEnabled("LOGGING") && opts.Logger != nil {
+		if scopedLogger != nil {
 			var staticLogAttrs []any
 			for _, attr := range staticAttrs {
 				staticLogAttrs = append(staticLogAttrs, slog.String(string(attr.Key), attr.Value.AsString()))
 			}
-			scopedLogger = opts.Logger.With(staticLogAttrs...)
+			scopedLogger = scopedLogger.With(staticLogAttrs...)
 		}
 	}
 	var otelOpts []otelgrpc.Option