[hardikkaurani]

Fixes #1544

Problem

In grpclog.LoggerV2, V(l int) bool is documented to report whether verbosity level l is at or below the configured verbose logging threshold. However, zapgrpc.Logger.V historically confused verbosity levels with zap's severity levels by mapping the verbosity int to a severity via _grpcToZapLevel[level]. This meant that verbose traces gated on V(l) were toggled purely based on the core's severity level instead of the intended verbosity level logic.

Solution

This PR adds an opt-in WithVerbosity(int) option that preserves backwards compatibility but allows callers to correctly switch to grpclog's documented verbosity semantics.

When WithVerbosity(verbosity) is provided during Logger construction:

• The internal useVerbosity flag is toggled on.
• Calls to V(level) will evaluate level <= verbosity rather than looking up severity levels.

If WithVerbosity is not provided, the default behaviour is preserved and V(l) maps to zapcore severity, ensuring no disruption for current users.

Testing

• Preserved existing test behaviour in TestLoggerV that verifies the fallback functionality.
• Added a new table-driven test TestLoggerVWithVerbosity that verifies verbosity evaluations check level <= verbosity independent of the underlying core severity level.

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[hardikkaurani]

@sywhang Just following up on #1544! This PR takes the safer, backwards-compatible approach (as discussed in #1546) by making the correct verbosity semantics strictly opt-in via a new \WithVerbosity(int)\ option. All tests are passing and it's ready for review when you have a moment. Thanks!