feat(spanner): add optional endpoint attribute to client metrics for location-aware routing

[rahul2393]

Summary

Adds an optional endpoint attribute to Spanner client metrics attempt-level instruments. When Spanner Omni location-aware routing selects a specific endpoint for a request, that endpoint is recorded as a metric attribute, so operators can attribute client-side attempt latency and attempt counts to the routed endpoint.

Scope

Stacked on top of #13741 (Client Metrics export). This PR adds only the endpoint attribute:

• ENDPOINT_KEY attribute plumbed through the custom-export views — attempt latency and attempt count only. Operation-level views and the Cloud Monitoring views deliberately drop it.
• KeyAwareChannel records the routed endpoint on the per-attempt tracer at endpoint-selection time.
• Tests covering endpoint presence on attempt metrics and its absence on operation, Cloud Monitoring, and default-client paths.

Stacking note

Until #13741 merges, this PR's diff also includes #13741's commit. Once #13741 lands, this branch will be rebased onto main for a clean endpoint-only diff. Do not merge before #13741.

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[gemini-code-assist]

Code Review

This pull request introduces support for exporting Cloud Spanner client-side metrics to a caller-owned OpenTelemetry destination. It adds the MetricsProvider interface and its implementations (DefaultMetricsProvider, NoopMetricsProvider, and CustomOpenTelemetryMetricsProvider), allowing custom metrics pipelines to be configured independently of the default Cloud Monitoring export. Additionally, it integrates endpoint routing labels for Spanner Omni and includes comprehensive test coverage. The reviewer feedback suggests a performance optimization in BuiltInMetricsConstant.java to pre-compute the common attribute keys as a static final field rather than streaming and mapping them on every call to baseAttributesFilter.

[gemini-code-assist]

The set of common attribute keys is derived from COMMON_ATTRIBUTES which is a static constant. Instead of streaming, mapping, and collecting COMMON_ATTRIBUTES on every single call to baseAttributesFilter, we can pre-compute this set of keys once as a static final field and reuse it. This improves both readability and performance during initialization.

  private static final Set<String> COMMON_ATTRIBUTE_KEYS =
      BuiltInMetricsConstant.COMMON_ATTRIBUTES.stream()
          .map(AttributeKey::getKey)
          .collect(Collectors.toSet());

  private static Set<String> baseAttributesFilter(boolean includeEndpointAttribute) {
    Set<String> attributesFilter = new HashSet<>(COMMON_ATTRIBUTE_KEYS);
    if (includeEndpointAttribute) {
      attributesFilter.add(ENDPOINT_KEY.getKey());
    }
    return attributesFilter;
  }