fix(gax): reduce visibility of ApiTracerContext methods

[blakeli0]

This PR

• Reduced visibility of several methods in ApiTracerContext to package-private to limit API surface area.
• Returns a new instance of OpenTelemetryMetricsFactory in withContext method to maintain immutability.

[gemini-code-assist]

Code Review

This pull request modifies ApiTracerContext.java by reducing the visibility of several methods from public to package-private. The review feedback correctly identifies these changes as breaking for the public API and recommends restoring the public access modifier for methods including serverPort(), operationType(), serviceName(), urlDomain(), destinationResourceId(), getAttemptAttributes(), empty(), and toBuilder().