feat: add addExporter to Meter

[dyladan]

Which problem is this PR solving?

• Adds a method to meter to register an exporter
• Required for SDK: Add Prometheus metrics exporter  #248

Short description of the changes

• Add addExporter(exporter: MetricExporter) to Meter
• Exporters are called each time a metric is updated. This is required to ensure the Prometheus exporter is up to date when its endpoint is called by the Metrics backend.

[codecov-io]

Codecov Report

Merging #482 into master will decrease coverage by <.01%.
The diff coverage is 96.61%.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   94.98%   94.98%   -0.01%     
==========================================
  Files         136      136              
  Lines        6482     6478       -4     
  Branches      567      567              
==========================================
- Hits         6157     6153       -4     
  Misses        325      325

Impacted Files	Coverage Δ
...kages/opentelemetry-metrics/test/mocks/Exporter.ts	100% <100%> (ø)
packages/opentelemetry-metrics/test/Meter.test.ts	100% <100%> (ø)	⬆️
packages/opentelemetry-metrics/src/Metric.ts	88.46% <100%> (ø)	⬆️
packages/opentelemetry-metrics/src/Utils.ts	100% <100%> (ø)	⬆️
packages/opentelemetry-metrics/src/Handle.ts	95.65% <100%> (+0.41%)	⬆️
packages/opentelemetry-metrics/src/Meter.ts	94.44% <89.47%> (-3%)	⬇️
...es/opentelemetry-node/src/instrumentation/utils.ts	90.47% <0%> (-9.53%)	⬇️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)	⬆️
...opentelemetry-base/test/resources/resource.test.ts	100% <0%> (ø)	⬆️
...ges/opentelemetry-exporter-zipkin/src/transform.ts	100% <0%> (ø)	⬆️
... and 15 more

[dyladan]

The only failing test is in @opentelemetry/plugin-dns and unrelated to this PR. Is there a way to rerun it or should I just ignore for now?

edit: it also only failed on node 11. Is there a flaky test?

edit again: now always failing?