feat(gax): implement dynamic channel refreshing on 401 retries

[blakeli0]

This PR implements dynamic channel refreshing on 401 Unauthenticated retries under the isMwlidEnvironment environment variable. It introduces compile-time type-safe refresh contracts across TransportChannel and ApiCallContext, with debouncing protection in ChannelPool to prevent connection stampedes.

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to automatically refresh transport channels when an UnauthenticatedException occurs, specifically within environments where the isMwlidEnvironment variable is set. Key changes include adding a refresh method to the TransportChannel interface and ChannelPool implementation, incorporating a 5-second debounce for refreshes, and updating the retry logic to trigger these refreshes. Review feedback highlights a potential bug in the debounce initialization, suggests using constants for magic numbers, recommends caching environment variable lookups to improve performance, and advises using imports instead of fully qualified names for better readability.

[gemini-code-assist]

Initializing lastRefreshTimeNanos to 0 can lead to the first refresh being skipped if System.nanoTime() returns a value close to zero (which is possible depending on the JVM's arbitrary time origin). Additionally, the 5-second debounce interval should be defined as a constant.

Suggested change

	private long lastRefreshTimeNanos = 0;
	private static final long REFRESH_DEBOUNCE_THRESHOLD_NANOS = java.util.concurrent.TimeUnit.SECONDS.toNanos(5);
	private long lastRefreshTimeNanos = System.nanoTime() - REFRESH_DEBOUNCE_THRESHOLD_NANOS;

[gemini-code-assist]

Use the newly defined constant for the debounce threshold instead of a hardcoded magic number.

Suggested change

	if (now - lastRefreshTimeNanos < TimeUnit.SECONDS.toNanos(5)) {
	if (now - lastRefreshTimeNanos < REFRESH_DEBOUNCE_THRESHOLD_NANOS) {

[gemini-code-assist]

Accessing environment variables via System.getenv on every RPC attempt (inside submit) is inefficient. Consider caching this value in a static final boolean field to avoid repeated lookups and potential performance overhead.

[gemini-code-assist]

The use of fully qualified names for UnauthenticatedException, ApiCallContext, and TransportChannel makes the code verbose and harder to read. It is recommended to use imports instead.

[gemini-code-assist]

The logic for checking the isMwlidEnvironment environment variable and the exception type is duplicated across both shouldRetry method overloads. Consider consolidating this into a private helper method and caching the environment variable result to improve maintainability and performance.