feat(Spanner): migrate the metrics exporter from a custom one to a native OTLP one.

[Hectorhammett]

Summary

In the past when we introduced client metrics, we implemented a custom exporter (MetricServiceClient) to translate metrics into Google Cloud Monitoring TimeSeries objects because GCP did not yet support the native OTLP protocol.

Google Cloud now natively supports OTLP metric ingestion at https://telemetry.googleapis.com/v1/metrics. This PR updates the Spanner PHP SDK built-in metrics exporter to use standard OTLP HTTP export and adds support for configurable metrics credentials.

Key Changes

• OTLP Exporter Migration: Replaced legacy MetricServiceClient GCM translation in MetricsExporter with standard OTLP HTTP exporter (OpenTelemetry\Contrib\Otlp\MetricExporter) targeting https://telemetry.googleapis.com/v1/metrics.
• Configurable & Re-scoped Credentials:
  • Added support for dedicated $options['metricsCredentials'] in SpannerClient.
  • Automatically re-scopes credentials to https://www.googleapis.com/auth/monitoring.write and https://www.googleapis.com/auth/cloud-platform using CredentialsWrapper to prevent OAuth scope mismatch errors.
  • Falls back gracefully to main client credentials or Application Default Credentials (ADC).
• Guzzle Authorization Middleware: Dynamically attaches Authorization: Bearer <token> and x-goog-user-project headers to outgoing OTLP HTTP requests via Guzzle middleware.