Auto-flush on process exit

[Mr0grog]

This is a bit of an ugly first pass. The code needs some cleanup and there are some open questions, but it works and has docs and tests. Fixes #136.

This attaches to the process’s beforeExit event (supported in Node.js, Deno, and Bun) when auto-flushing is enabled in order to automatically flush when a program exits. Before, you needed to remember to call metrics.flush() manually at the end of your program, which is easy to forget (or not even know about in the first place!).

It also adds a close() method (not sure about the name) that flushes and buffered metrics and disables auto-flushing. It does not prevent adding new metrics to the buffer, but maybe it should? (Would they error? Get dropped silently? Call the onError handler?)

To-do/open questions:

• Clean up implementation. At the very least, get some of this code out of the BufferedMetricsLogger constructor.
• Dedupe all the mock reporter code in tests.
• Is close() a good name? Other ideas: (Update: renamed to stop)
  • stop()
  • shutdown()
  • destroy()
  • disable()
  • pause()
• Should close() also prevent buffering new metrics? (i.e. make addPoint() a no-op, or maybe throw an exception or call the global error handler) (Update: decided no for now.)

[Mr0grog]

Cleaned this up a bit and it should be good to go.

Besides burying too much code in the constructor, I think my initial implementation was a bit too fancy by trying to retain a subtle previous behavior where you could adjust auto-flushing on the fly by changing loggerInstance.flushIntervalSeconds, which would be picked up on the next flush. That property was already private though, and I don’t think that usage was ever really intended. Doing this also made it easier to rename close() to stop() (since it now has a twin start() method, which makes things feel a bit more well aligned to me).

Since renaming the method to stop(), I think that also answers the final question about disabling new metrics — “stop” doesn’t really imply a total shutdown, so it doesn’t disable anything other than auto-flushing (which can now be restarted if you want).