refactor(metrics): distinguish different aggregator types

[legendecas]

Which problem is this PR solving?

Separate the toPoint definition to distinguish the return value of one aggregator possibly returning.

The change made to Aggregator is that one aggregator is determined to return one type of point value type.

Currently, the Aggregator definition returns any one of the point value types in a single aggregator, i.e. an aggregator can return a LastValue and a Histogram for each toPoint call since they all conform to the signature.

const metricRecord: MetricRecord = getRecordSomeWhere()

if (metricRecord.aggregator.kind === AggregatorKind.HISTOGRAM) {
  const point = metricRecord.aggregator.toPoint()
  // this point is infered as Point<PointValueType>, so type casting is still needed.
  // accessing `point.value.count` needs a type casting
}

The solution proposed in the PR made an aggregator can only return a definite subset of PointValueType. i.e. an aggregator with HistogramAggregatorType returning a LastValue is not going to pass the type check. This made the aggregator kind check prominent and enforced.

const metricRecord: MetricRecord = getRecordSomeWhere()

if (metricRecord.aggregator.kind === AggregatorKind.HISTOGRAM) {
  const point = metricRecord.aggregator.toPoint()
  // this point can be infered as Point<Histogram> without any manual type casting 
  // since aggregator can be determined as type of SumAggregatorType for
  //  `metricRecord.aggregator.kind === AggregatorKind.HISTOGRAM` branching.
}

Short description of the changes

Split Aggregator type definition to several concrete AggregatorTypes, like SumAggregatorType and HistogramAggregatorType.

[codecov]

Codecov Report

Merging #1325 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1325      +/-   ##
==========================================
+ Coverage   93.61%   93.63%   +0.01%     
==========================================
  Files         149      149              
  Lines        4261     4272      +11     
  Branches      870      871       +1     
==========================================
+ Hits         3989     4000      +11     
  Misses        272      272              

Impacted Files	Coverage Δ
...emetry-metrics/src/export/aggregators/Histogram.ts	100.00% <100.00%> (ø)
...trics/src/export/aggregators/MinMaxLastSumCount.ts	100.00% <100.00%> (ø)
...pentelemetry-metrics/src/export/aggregators/Sum.ts	100.00% <100.00%> (ø)
packages/opentelemetry-metrics/src/export/types.ts	100.00% <100.00%> (ø)

[vmarchaud]

👍

[legendecas]

@open-telemetry/javascript-maintainers hey folks, this PR has been open for 17 days, and one more approval is required to merge this PR. However, there is a statement on the document that "A pull request opened by an approver may be merged with only 3 reviews.". If you think this PR looks good, please find a time to merge it :)