channelz: log on channelz trace events and trace on channelz relevant logs.

[GarrettGutierrez1]

Change to log (using grpclog) on channelz trace events and generate channelz trace events on grpclogs. Only grpclogs in proximity of / relevant to a channelz channel were modified.

updates #2939

[dfawley]

This approach seems fine. Some comments inline.

@menghanl noticed that the depth may pretty easily be a constant when we bring in module-level logging. If we do all our logging/channelz-ing through module-level wrappers, we can probably accomplish this fairly easily & naturally.

[dfawley]

Remove LoggerV2 here; add to comment something like:

// ...  If a logger implements DepthLogger, the below functions will be called with
// the appropriate stack depth set for trivial functions the logger may ignore.
//
// This API is EXPERIMENTAL.

[dfawley]

Let's move these to internal/grpclog so they are not visible to users.

To avoid the circular dependency (grpclog depends on internal/grpclog and vice-versa) and also avoid the type assertion where this is used:

package internal/grpclog:

• Copy DepthLogger and V2Logger interfaces here (unfortunate but this is internal). Type aliases would be better, but those will make the godoc of grpclog bad.
• Add globals: var DepthLogger DepthLogger and var Logger V2Logger.
• Add func InfoDepth/etc, using DepthLogger if non-nil, or calling Logger.Info/etc otherwise.
  • Our packages call these functions instead of via the grpc/grpclog package.

package grpclog: imports internal/grpclog

• Remove global logger.
• Set internal/grpclog.Logger and internal/grpclog.DepthLogger together, clearing the DepthLogger if the V2Logger does not implement it.
• Use internal/grpclog.Logger instead of the global logger.