docs: add a section for using metrics to the latency_debugging_guide

[olavloite]

The current latency_debugging_guide only contains information about tracing. The OpenTelemetry integration however also includes metrics, and while these are not directly useful for investigating the latency of a single statement, they are useful for monitoring the overall health of the system.
The lack of this documentation also means that it is easy for users to miss the requirement to call SpannerOptions.enableOpenTelemetryMetrics() in order to use metrics with JDBC.