[Prometheus Emitter] Add to code coverage and remove code smell

[ashwintumma23]

Description

Added code coverage ...

• Additions to cover the error and exception cases for Prometheus emitter's metrics

Fixed a code smell ...

• Same variable name, registeredMetrics was being used for Class variable name as well as local variable where configs are being parsed
• Renamed the local variable to parsedRegisteredMetrics to reflect the local map's purpose.
• Why was marked as a Code smell?
  • Shadowing the variable in outer scope impacted readability, and hence the maintainability of that specific block of code

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Key changed/added classes in this PR

• Metrics
• MetricsTest

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This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[cryptoe]

Thank you for the contribution @ashwintumma23 🚀