feat: Add optional keepAlive option to Meter and Timer

[tuckbick]

This PR unrefs timers by default, eliminating headaches for people unsure about why their unit tests keep hanging.

It also eliminates having to break a 1 line piece of code into 3:

// no unref-ing
collection.meter('responses').mark()

// with unref-ing
const responsesMeter = collection.meter('responses');
responsesMeter.unref();
responsesMeter.mark();

// with PR, keepAlive = false
collection.meter('responses').mark()

// with PR, keepAlive = true
collection.meter('responses', {keepAlive: true}).mark()

I've set the default to false since that was most helpful for my team. However, I could be convinced to have it default to true.

[tuckbick]

Prettier made this change.

[coveralls]

Pull Request Test Coverage Report for Build 374

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 89.042%

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Totals
Change from base Build 372:	0.4%
Covered Lines:	759
Relevant Lines:	835

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💛 - Coveralls

[fieldju]

What are the consequences of making this the default behavior rather than inverting this logic?
Is this a breaking change for people?

[fieldju]

I mean having the default be false.

[csabapalfi]

In an application or process that has any kind of active handle (e.g. listening on a port, other timer etc) that'll keep the event loop alive anyway.

If measured-core is used in one-off jobs or similar then this behavior usually causes issues and keeps the process hanging.

I see it as highly unlikely to be breaking.

If you see this as a problem I can certainly unpublish 1.43.0 and release this as a major but to me this very much is a minor.