grpclog: refactor existing logger interfaces for better interoparability

[easwars]

PrefixLogger is a concrete type, which has a field logger of type grpclog.DepthLoggerV2. The former delegates all logging to the latter.

grpclog.DepthLoggerV2 does not have a Debug or V method.

type DepthLoggerV2 interface {
	// InfoDepth logs to INFO log at the specified depth. Arguments are handled in the manner of fmt.Println.
	InfoDepth(depth int, args ...interface{})
	// WarningDepth logs to WARNING log at the specified depth. Arguments are handled in the manner of fmt.Println.
	WarningDepth(depth int, args ...interface{})
	// ErrorDepth logs to ERROR log at the specified depth. Arguments are handled in the manner of fmt.Println.
	ErrorDepth(depth int, args ...interface{})
	// FatalDepth logs to FATAL log at the specified depth. Arguments are handled in the manner of fmt.Println.
	FatalDepth(depth int, args ...interface{})
}

The global Logger, which we have to end up using (to implement debug logging and verbosity checking) is of type grpclog.LoggerV2, and it has methods for the same. We need to refactor these interfaces a little to ensure the implementation of PrefixLogger can delegate all calls to its field logger instead of having to use the package global Logger.

[easwars]

Fixed by #7465.