Reduce overhead of metrics recording

[njhill]

This is an optimization to reduce the amount of objects allocated when the various metrics are recorded, and minimize processing done on the application threads.

The various events are now passed in "raw" form via dedicated objects to the sender thread (rather than first being serialized to a string on the app thread). All of the serialization work is now done on the sender thread, avoiding allocation of any new strings or byte arrays. A single StringBuilder is reused in conjunction with the existing ByteBuffer. The NumberFormat ThreadLocals are no longer needed since they will now only be used by the one thread.

Also included are a couple of thread-safety tweaks to the unit tests.

[njhill]

Now re-based on latest master branch. Any interest in this change?

[darxriggs]

@njhill there are merge conflicts that should be resolved.
(I am not the maintainer.)

[njhill]

@darxriggs rebasing isn't completely trivial and I don't want to waste effort doing it if there's no interest in the PR, which appears to be the case.

[truthbk]

@njhill unfortunately this PR didn't get the attention it deserved. I apologize for the time it has taken us to provide any reasonable feedback for it.

We would be very interested in helping drive this PR forward as it would be of great value to us and the community, helping reduce both memory and cpu overhead. There's a bunch of ongoing work as well, but I'd like to give this precedence as the ongoing work will introduce some breaking changes, and trigger a new major release of the library. Also, I'd like to rebase that on top of this, and not the other way around.

I'm not sure if you'd have time to rebase, or if you'd like us to take over and finish any additional work required (all credit would still go to you, of course). Please let us know.

In any case, thank you very much for this great PR, it's very welcome and definitely of help 🙇

[njhill]

Thanks @truthbk, I'm not sure when I'd have time to rebase (and actually am not using the client any longer anyhow), so feel free to take that over if you need it done soon!

[truthbk]

@njhill sounds good! I'll take over the PR! I might close this one and bring your changes into a branch on this repo (and not your fork). In any case your commits and authorship will be preserved. Thank you very much for this work.

[truthbk]

Closing in favor of #105, thank you @njhill - rebased your contribution there.