feat: improve metadata observability

[Rachit-Gandhi]

Summary

• add contextual wrapping for metadata service-name mapping register/get/remove failures
• publish and collect metadata mapping register/get/remove metrics from DelegateMetadataReport
• add MetadataError/MetadataErrorKind so metadata report, RPC, URL-build, and nil-metadata failures are categorizable with context

Verification

• go test ./metadata/mapping/metadata ./metadata ./metrics/metadata ./registry/servicediscovery -count=1
• go test ./...

Relates to #3356

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
8.8% Duplication on New Code

See analysis details on SonarQube Cloud