Add OTLP trace export and bump spannerotel to v0.2.0

[apstndb]

Summary

• Add --experimental-trace-otlp and --experimental-trace-otlp-endpoint for local OpenTelemetry collectors (xor with Cloud Trace and stderr export).
• Bump spannerotel to v0.2.0 (OTLP export, service.name resource).
• Expand README tracing docs (PROFILE plan spans, exporter matrix).

Test plan

• go test ./...
• Integration test covers tracing without OpenCensus bridge

Made with Cursor

[gemini-code-assist]

Code Review

This pull request adds support for exporting OpenTelemetry spans via OTLP/gRPC to a local collector, introducing the --experimental-trace-otlp and --experimental-trace-otlp-endpoint CLI flags, updating the documentation, and upgrading dependencies. A critical issue was identified in trace.go where checking n != 1 for mutually exclusive flags prevents the optional case (where no tracing flags are set) from falling through to the default configuration, incorrectly triggering an error instead.

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[gemini-code-assist]

By checking n != 1, any case where no tracing flags are set (n == 0) will trigger the error exactly one of ... must be set. This makes the default branch in the switch statement (which returns tracing is not configured) completely unreachable.

Additionally, if traceConfig is called when tracing is disabled, the error message claiming that one of the flags must be set is misleading. Changing the check to n > 1 allows the n == 0 case to correctly fall through to the default branch.

Suggested change

	if n != 1 {
	return tracing.Config{}, fmt.Errorf("exactly one of --experimental-trace-otlp, --experimental-trace-stdout, or --experimental-trace-project must be set")
	}
	if n > 1 {
	return tracing.Config{}, fmt.Errorf("at most one of --experimental-trace-otlp, --experimental-trace-stdout, or --experimental-trace-project can be set")
	}

References

1. When configuring mutually exclusive CLI flags, omit the required constraint if the flags are optional as a group, allowing the user to specify none of them.