chore(spanner): add client identity and monitored resource attributes to built-in metrics

[olavloite]

• Add parse_database_name, generate_client_uid, generate_client_hash, and client_name helper functions for Spanner built-in metrics.
• Attach an OpenTelemetry Resource (project_id, instance_id, location, instance_config, client_hash) to SdkMeterProvider in Observability::init so metrics export with correct spanner_instance resource labels.
• Pre-construct common metric attributes (client_uid, client_name, database) on the Observability struct and include them on operation and attempt recordings without repeated string cloning.
• Add comprehensive unit tests for database name parsing, deterministic client hashing, and client UID/name formatting.

[gemini-code-assist]

Code Review

This pull request introduces client-side metrics for Spanner observability by parsing database components, generating unique client identifiers (client_uid and client_hash), and attaching common attributes to metrics. Feedback suggests improving the client_hash calculation to use the 24 least significant bits of the FNV-1a hash to reduce collisions, and optimizing client_name to return a static string slice (&'static str) using the concat! macro to avoid unnecessary runtime allocations.

[codecov]

Codecov Report

❌ Patch coverage is 96.92308% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.06%. Comparing base (57dae51) to head (3ffbd7d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/observability/metrics.rs	96.89%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6245    +/-   ##
========================================
  Coverage   96.05%   96.06%            
========================================
  Files         271      271            
  Lines       68537    68649   +112     
========================================
+ Hits        65835    65946   +111     
- Misses       2702     2703     +1     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces experimental built-in metrics support for the Spanner client by adding the uuid dependency and implementing helper functions to parse database names, generate unique client identifiers, and compute client hashes. The Observability struct is updated to initialize and attach these common attributes to recorded metrics, supported by new unit tests. There are no review comments, and the changes conform to the repository's style guidelines.