perf(spanner): create Cloud Monitoring MetricServiceClient lazily

[fornwall]

Issue (review P1-6)

SpannerCloudMonitoringExporter.create() eagerly called MetricServiceClient.create(...), which builds a second full GAPIC client — its own gRPC channel, executor and watchdog — dedicated to Cloud Monitoring.

This runs synchronously inside BuiltInMetricsProvider.getOrCreateOpenTelemetry(), which is invoked from the GapicSpannerRpc constructor (getFallbackOpenTelemetry → SpannerOptions.getBuiltInOpenTelemetry). So every Spanner client stands up the whole monitoring client stack before the Spanner stub is usable — even though the first metric export only happens after the first reader interval (~1 minute later). Pure wasted startup latency and thread/channel allocation.

I traced the path against source to confirm the finding is real:
GapicSpannerRpc ctor → getFallbackOpenTelemetry → SpannerOptions.getBuiltInOpenTelemetry → BuiltInMetricsProvider.getOrCreateOpenTelemetry → SpannerCloudMonitoringExporter.create → MetricServiceClient.create.

Fix

Defer building the MetricServiceClient to the first non-empty export():

• create() now only builds the (cheap) MetricServiceSettings and hands the exporter a MetricServiceClientFactory that constructs the client on demand.
• getOrCreateClient() creates it exactly once under a lock. export() returns early for empty collections, so idle reader intervals never create the client.
• shutdown() is now idempotent via a dedicated shutdown flag and skips work entirely when the client was never created (returns success — nothing to tear down).
• The heavy work (channel/executor/watchdog) is what's deferred; building settings stays eager, so getMetricServiceClient().getSettings() (used by testUniverseDomain) still works by forcing creation.

Chose the lazy-client approach over "build the OTel stack on a background thread" because it's fully contained within the exporter, avoids new background-thread ordering/visibility concerns, and directly targets the expensive part.

Tests

• testClientCreatedLazilyOnFirstNonEmptyExport — asserts the client is not created at construction, not created by an empty export, created exactly once on the first non-empty export, and reused thereafter.
• testShutdownBeforeExportDoesNotCreateClient — asserts shutdown before any export succeeds without creating a client, is idempotent, and a post-shutdown export fails without creating one.

All 13 tests in SpannerCloudMonitoringExporterTest pass. fmt:check clean.

Not addressed (related P2)

The review bundles a P2 note: the gRPC built-in-metrics path (SpannerOptions.enablegRPCMetrics) is not gated on emulator/NoCredentials, so emulator/CI runs still build the OTel meter provider despite never being able to export. That's a separate behavioral change and is left as a follow-up.

🤖 Generated with Claude Code

https://claude.ai/code/session_016Ci2KFsNgWdHCno4dkXdUU