Migrate from New Relic go-agent v2 to v3

[AmanGIT07]

pkg/db/db.go and internal/store/spicedb/relation_repository.go import the deprecated github.com/newrelic/go-agent v2 package. Migrate to v3: new import path and the reworked transaction/segment APIs at every telemetry call site.

Part of #1782.

[ravisuhag]

@AmanGIT07 we can move away from new relic to open telemetry.

[AmanGIT07]

Noted, will check it out @ravisuhag

[AmanGIT07]

Looked into this. The v2 agent turns out to be dead code: nothing ever initializes a New Relic application and nothing reads the new_relic config, so newrelic.FromContext always returns nil and the datastore segments never run.

So instead of porting the call sites to v3, #1876 removes the dependency and its call sites entirely.

For the OpenTelemetry direction suggested above: opened #1875 to add OTel tracing on top of the existing otelconnect setup, which already collects RPC metrics with tracing currently disabled.