Support multiple reporter instances

[Mr0grog]

While working on #80, I noticed that you can’t instantiate multiple instances of DataDogReporter (or, you can, but they will behave the same, and creating a reporter will alter any that you have already made). This is mainly because the underlying metricsApi object is scoped to the whole module, rather than to each instance of DataDogReporter.

This prevents someone from having two loggers with different credentials (probably not a common case, but it ought to work), and will be a bigger problem if we ever lean on configuration built-into the @datadog/datadog-api-client package for more than just credentials and what site you are submitting to.

I assume the reason for this was to keep access private. There are a couple instance-specific ways to do this:

• Use WeakMap. Adds a little more boilerplate, but is supported in all version of Node.js we are compatible with.
• Use private class fields. These are much nicer, syntax-wise, but I believe requires bumping the minimum version of Node.js to 14 (and switching to the newer class syntax, which we should probably do anyway).