feat(storage): add stream open latency metrics and trace annotations

[kalragauri]

This PR captures microsecond timestamps across connection establishment, request dispatch, and metadata response receipt, exporting them to OpenTelemetry histograms and adding trace span event annotations. Follow-up PRs will add metrics for other required paths.

[gemini-code-assist]

Code Review

This pull request introduces OpenTelemetry metrics and tracing to track network handshake, server metadata, and end-to-end stream open latencies in OpenObject. The review feedback highlights several critical issues regarding conditional compilation: using an invalid OpenTelemetry API call instead of the SDK's internal CurrentSpan() function, and unconditionally declaring/assigning time-tracking variables (t0_, t1_, t2_, and t3) which introduces unnecessary overhead and triggers compiler warnings when metrics are disabled. Wrapping these variables and assignments in the appropriate preprocessor guards is recommended.

[kalragauri]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces latency metrics and tracing span events for asynchronous open object operations by adding the OpenObjectMetrics class and integrating it into OpenObject. The review feedback correctly identifies that tracing span events are currently tightly coupled with the metrics compilation flag (GOOGLE_CLOUD_CPP_STORAGE_WITH_OTEL_METRICS). To ensure tracing works independently of metrics, the reviewer suggests decoupling the timestamp tracking and span event recording so they are active when either tracing (GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY) or metrics is enabled.