Refactor for DataValue

[victlu]

Refactor out value portion of IDataPoint to a IDataValue.

[codecov]

Codecov Report

Merging #2093 (060d29d) into metrics (ed3322c) will decrease coverage by 0.04%.
The diff coverage is 30.64%.

@@             Coverage Diff             @@
##           metrics    #2093      +/-   ##
===========================================
- Coverage    83.00%   82.96%   -0.05%     
===========================================
  Files          209      211       +2     
  Lines         6698     6694       -4     
===========================================
- Hits          5560     5554       -6     
- Misses        1138     1140       +2     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/DataPoint/DataPoint.cs	0.00% <0.00%> (-37.50%)	⬇️
...rc/OpenTelemetry/Metrics/DataPoint/DataPoint{T}.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/Exemplar.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/Exemplar{T}.cs	0.00% <0.00%> (ø)
src/OpenTelemetry/Metrics/DataPoint/DataValue.cs	55.55% <55.55%> (ø)
...y/Metrics/MetricAggregators/SumMetricAggregator.cs	80.51% <71.42%> (+1.85%)	⬆️
...rc/OpenTelemetry/Metrics/DataPoint/DataValue{T}.cs	100.00% <100.00%> (ø)
...Metrics/MetricAggregators/GaugeMetricAggregator.cs	100.00% <100.00%> (+27.50%)	⬆️
...emetry/Metrics/Processors/MetricConsoleExporter.cs	88.00% <100.00%> (-4.00%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
... and 6 more

[pirgeo]

LGTM, I just have one question: In the proto and the spec, Attribute or Label is used as the name for Key-Value pairs, whereas here, Tag is used - I might be comparing apples to oranges here, so if I do, please let me know, but would it make sense to keep the naming consistent everywhere as much as possible? Or is there some guidance on when to use what that that I simply missed?

[victlu]

@pirgeo You are correct. We need to rename tag to attribute. Just haven't gotten to it yet.