metrics/json: extract common JSON marshaling logic to eliminate duplication

[prestoalvarez]

Extract shared JSON marshaling logic into a private helper function to eliminate code duplication between StandardRegistry.MarshalJSON and PrefixedRegistry.MarshalJSON.

• Both registry types had identical MarshalJSON implementations.
• Duplication increases maintenance burden and risk of inconsistencies.
• Common logic should be centralized to follow DRY principles.

• Add private marshalRegistryJSON(r Registry) helper function.
• Update both MarshalJSON methods to use the shared helper.

[fjl]

We prefer not to include this change.