add methods to gauge-track the in-flight count of functions and promises

[dt-rush]

As suggested @ https://github.com/balena-io/balena-builder/pull/707#discussion_r388613618

Change-type: minor
Signed-off-by: dt-rush [email protected]

[dt-rush]

Odd that this passed on my local:

15 passing (28ms)
  1 failing

  1) Gauge
       should track runtime of promises properly:
     TypeError: promise.finally is not a function
      at MetricsGatherer.resolve (src/metrics-gatherer.ts:125:18)
      at Context.it (test/index.spec.ts:67:11)

[Page-]

Can you change this to

Suggested change

	setTimeout(() => {
	output = metrics.output();
	expect(/promise_running_gauge 0/.test(output)).to.be.true;
	}, 200);
	await Bluebird.delay(200);
	output = metrics.output();
	expect(/promise_running_gauge 0/.test(output)).to.be.true;

as it's clearer and also will mean that if the expect(/promise_running_gauge 0/.test(output)).to.be.true fails then it will be handled properly. The same goes for other setTimeouts

[Page-]

This is better as

Suggested change

	promise.finally(() => {
	this.inc(name, -1, labels);
	});
	try {
	await promise
	} finally {
	this.inc(name, -1, labels);
	}

because it should also handle promises that don't support .finally

[Page-]

Matching the other function this should be

Suggested change

	await func();
	this.inc(name, -1, labels);
	try {
	await func();
	} finally {
	this.inc(name, -1, labels);
	}

and make sure we don't drop decrements