Add OpenTelemetry based metrics to Spanner client

[dario-oai]

• We are using the SpannerClient at scale in production and wanted more insights into some of the internal metrics to ensure things are running smoothly and understand which config knobs to tweak if needed
• I have confirmed metric emissions for most cases, but would still need to check get session timeout
• The otel-metrics feature cfg enablement is pretty verbose, but not sure if this should be opt in with a feature flag or as a default. I could refactor to make it less verbose, but if it's okay to add opentelemetry as a dependency, most of the code could be removed. Please let me know your preference
  • I can add doc strings after deciding how this should be added
• Use the Go library implementation as reference