fix: spanner metrics regression

[vroldanbet]

Description

When using --datastore-spanner-metrics=otel, none of the Spanner metrics were being emmitted. This PR fixes that.

The SDK has deprecated OpenCensus support, which was wired into the prometheus endpoint, in favour of OpenTelemetry. This was reflected in a recent SpiceDB change, which made the otel approach the default, and as a consequence all prometheus metrics disappeared.

This commit adds the prometheus exporter as a OpenTelemetry MetricsResolver, to add some the metrics back. The naming convention has changed, and I'm not aware of a way around that.

Also client-side latency metrics are not supported via the opentelemetry integration. See https://cloud.google.com/spanner/docs/capture-custom-metrics-opentelemetry#capture-client-round-trip-latency Instead they now have to be manually instrumented.

Testing

Try docker-compose -f docker-compose.spanner.yaml up --build and go to http://localhost:3000/. You should see the metrics:

spanner_gfe_header_missing_count_total
spanner_max_allowed_sessions_ratio
spanner_max_in_use_sessions_ratio
spanner_num_acquired_sessions_total
spanner_num_released_sessions_total
spanner_num_sessions_in_pool_ratio
spanner_open_session_count_ratio

If you set in the Docker-compose file --datastore-spanner-metrics=deprecated-prometheus, you get the same metrics but with the prefix spicedb_cloud_google_com_go_.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.59%. Comparing base (c3b1bdc) to head (cfc12dc).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/datastore/spanner/spanner.go	58.34%	17 Missing and 3 partials ⚠️

❌ Your project status has failed because the head coverage (74.59%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2329      +/-   ##
==========================================
+ Coverage   74.54%   74.59%   +0.05%     
==========================================
  Files         484      484              
  Lines       59201    59228      +27     
==========================================
+ Hits        44126    44173      +47     
+ Misses      11982    11969      -13     
+ Partials     3093     3086       -7     

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[tstirrat15]

LGTM