Fix prometheus exporter to handle histogram with no buckets

[cijothomas]

Fixes #.

Changes

Please provide a brief description of the changes here.

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 #2651 (9e3d78d) into main (31a2d67) will increase coverage by 0.01%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##             main    #2651      +/-   ##
==========================================
+ Coverage   81.31%   81.33%   +0.01%     
==========================================
  Files         245      246       +1     
  Lines        8645     8659      +14     
==========================================
+ Hits         7030     7043      +13     
- Misses       1615     1616       +1     

Impacted Files	Coverage Δ
...tryProtocol/Implementation/MetricItemExtensions.cs	50.00% <42.85%> (-0.71%)	⬇️
...ometheus/Implementation/PrometheusSerializerExt.cs	100.00% <100.00%> (ø)
src/OpenTelemetry/Metrics/MetricPoint.cs	90.51% <100.00%> (-0.24%)	⬇️
src/OpenTelemetry/ReadOnlyTagCollection.cs	100.00% <100.00%> (ø)
...ter.ZPages/Implementation/ZPagesActivityTracker.cs	97.14% <0.00%> (-2.86%)	⬇️

[CodeBlanch]

LGTM