fix: do not clear other labelsets when updating metrics

[dyladan]

Originally reported by @obecny in gitter:

The following code:

'use strict';

const { PrometheusExporter } = require('@opentelemetry/exporter-prometheus');
const { MeterProvider } = require('@opentelemetry/metrics');
const exporter = new PrometheusExporter(
  {
    startServer: true,
  },
  () => {
    console.log('prometheus scrape endpoint: http://localhost:9464/metrics');
  },
);

const meter = new MeterProvider({
  interval: 5000,
  exporter: exporter,
}).getMeter('foo');


const counter = meter.createCounter('foo', {
  monotonic: true,
  labelKeys: ['name'],
  description: 'foo',
});

const boundCounter1 = counter.bind({ name: 'foo1' });
const boundCounter2 = counter.bind({ name: 'foo2' });

boundCounter1.add(12);
boundCounter2.add(15);

Gave the following prometheus export:

# HELP foo foo
# TYPE foo counter
foo{name="foo2"} 15

Expected:

# HELP foo foo
# TYPE foo counter
foo{name="foo1"} 12
foo{name="foo2"} 15

This was caused by two issues:

1. in the batcher, there is no longer a labelset identifier that can be used to identify a batch
2. in the exporter, clearing the metric cleared it for all labels

[obecny]

can you add some unit test for that ?

[codecov-io]

Codecov Report

Merging #941 into master will decrease coverage by 0.00%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #941      +/-   ##
==========================================
- Coverage   94.94%   94.94%   -0.01%     
==========================================
  Files         247      248       +1     
  Lines       11142    11173      +31     
  Branches     1065     1068       +3     
==========================================
+ Hits        10579    10608      +29     
- Misses        563      565       +2     

Impacted Files	Coverage Δ
...ackages/opentelemetry-metrics/test/Batcher.test.ts	92.59% <92.59%> (ø)
...pentelemetry-exporter-prometheus/src/prometheus.ts	92.07% <100.00%> (+0.07%)	⬆️
...emetry-exporter-prometheus/test/prometheus.test.ts	98.70% <100.00%> (+<0.01%)	⬆️
...ckages/opentelemetry-metrics/src/export/Batcher.ts	100.00% <100.00%> (ø)

[dyladan]

@obecny @mayurkale22 added tests

[mayurkale22]

Great :)

[obecny]

lgtm