fix: do not register CRDB metrics in init()

[miparnisari]

Description

Before: if you spun up SpiceDB with a datastore OTHER THAN CRDB, you could see CRDB metrics published.

Now: you don't see them.

Testing

docker-compose up --build and then explore http://localhost:3000/.

[codecov]

Codecov Report

❌ Patch coverage is 72.41379% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.02%. Comparing base (0e16dd1) to head (eedc13b).

Files with missing lines	Patch %	Lines
...nal/datastore/proxy/schemacaching/watchingcache.go	0.00%	8 Missing ⚠️
internal/datastore/crdb/pool/balancer.go	85.30%	5 Missing ⚠️
internal/datastore/crdb/pool/health.go	75.00%	3 Missing and 1 partial ⚠️
internal/datastore/crdb/pool/pool.go	73.34%	3 Missing and 1 partial ⚠️
internal/datastore/crdb/crdb.go	78.58%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2966      +/-   ##
==========================================
- Coverage   48.88%   46.02%   -2.86%     
==========================================
  Files         429      429              
  Lines       54628    54687      +59     
==========================================
- Hits        26700    25163    -1537     
- Misses      25220    26881    +1661     
+ Partials     2708     2643      -65     

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