minor: add getServiceDims() getter to ServiceMetricEvent

[abhishekrb19]

Summary

Adds a getServiceDims() getter method to ServiceMetricEvent to provide direct access to the immutable service dimensions map. This can be used by custom extensions along with the existing getters in the class.

Motivation

Previously, callers could access individual service dimension values via getService() and getHost(), but had no way to retrieve the complete service dimensions map. This getter provides direct access to the immutable map of all service
dimensions by custom extensions.

This PR has:

• been self-reviewed.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.

[utafrali]

The getter is functionally correct and well-tested, but returning ImmutableMap<String, String> is inconsistent with the existing getUserDims() which returns the Map interface — both fields are backed by ImmutableMap, yet the API surface is different. The return type should be Map<String, String> to keep the class API coherent.

[utafrali]

The return type ImmutableMap<String, String> leaks the concrete implementation, which is inconsistent with getUserDims() in the same class — that method returns Map<String, Object> even though its backing field is also an ImmutableMap. For a public API, prefer returning the interface type:

public Map<String, String> getServiceDims()
{
  return serviceDims;
}

Callers who need the immutability guarantee can check instanceof ImmutableMap or the Javadoc can document it. Returning the concrete type here makes it harder to swap the implementation later.

[abhishekrb19]

Yes, that's a good point - thanks for calling that out!

[utafrali]

The string literals "service" and "host" duplicate the key constants already defined in the class (used inside getService() and getHost()). If those constants (SERVICE, HOST) are accessible from the test, use them directly to avoid silent drift if the keys ever change:

Assert.assertEquals("test", builderEvent.getServiceDims().get(ServiceMetricEvent.SERVICE));
Assert.assertEquals("localhost", builderEvent.getServiceDims().get(ServiceMetricEvent.HOST));

If the constants are private, at minimum add a comment explaining why literals are used here.

[abhishekrb19]

“service” and “host” are documented metric dimensions. Hence, they cannot change due to backward compatibility guarantees, and this test and other existing ones that assert on the literals would catch that unlikely drift.