Extract a Subscriber base class from LogSubscriber

I'm using the instrumentation framework heavily to do stats collection with Statsd, but have so far relied on a custom base class for attaching the various subscribers to instrument names. I'd like to use the same base class for both stats and log subscribers, hence this PR.

There should be no functional changes to how the LogSubscriber works, but now you can inherit from Subscriber if you just want the attachment logic.

What do you say?