Add a value type version of Batch<T>

[reyang]

Related to #1315 and #1325.

To make it easier, please review the 2nd commit which has the diff (the 1st commit is a direct copy of the existing Batch).

Changes

• Introduced a BatchStruct for value types, which helps to avoid heap allocation.
• Added stress test for both the reference type and value type for comparison.
• On my machine I have observed 40% perf increase using the value type (BatchStruct).

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1327 into master will decrease coverage by 0.46%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1327      +/-   ##
==========================================
- Coverage   76.51%   76.04%   -0.47%     
==========================================
  Files         224      225       +1     
  Lines        6471     6507      +36     
==========================================
- Hits         4951     4948       -3     
- Misses       1520     1559      +39     

Impacted Files	Coverage Δ
src/OpenTelemetry/BatchStruct.cs	0.00% <0.00%> (ø)
...us/Implementation/PrometheusExporterEventSource.cs	63.63% <0.00%> (-9.10%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	56.25% <0.00%> (-6.25%)	⬇️
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️

[reyang]

Superseded by #1328.