[Instrumentation.Runtime] Enable nullable

[Kielek]

Towards #894

Changes

Nullable for Runtime projects.

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

• [ ] Appropriate CHANGELOG.md updated for non-trivial changes
• [ ] Design discussion issue #

[codecov]

Codecov Report

Merging #893 (94e875d) into main (b53b6b9) will decrease coverage by 0.06%.
The diff coverage is 81.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #893      +/-   ##
==========================================
- Coverage   68.48%   68.42%   -0.06%     
==========================================
  Files         183      183              
  Lines        7002     7006       +4     
==========================================
- Hits         4795     4794       -1     
- Misses       2207     2212       +5     

Impacted Files	Coverage Δ
...entation.Process/MeterProviderBuilderExtensions.cs	85.71% <50.00%> (-14.29%)	⬇️
...entation.Runtime/MeterProviderBuilderExtensions.cs	87.50% <50.00%> (-12.50%)	⬇️
...elemetry.Instrumentation.Runtime/RuntimeMetrics.cs	100.00% <100.00%> (ø)
...trumentation.EventCounters/EventCountersMetrics.cs	75.90% <0.00%> (-3.62%)	⬇️

[Kielek]

Fix for failing test potentially in #897