Use Milliseconds instead of Millis

[reyang]

Follow up #1135 (comment).

Changes

Replaced all the remaining "Millis" with "Milliseconds".

[codecov]

Codecov Report

Merging #1139 into master will decrease coverage by 0.04%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master    #1139      +/-   ##
==========================================
- Coverage   77.28%   77.23%   -0.05%     
==========================================
  Files         220      220              
  Lines        6269     6269              
==========================================
- Hits         4845     4842       -3     
- Misses       1424     1427       +3     

Impacted Files	Coverage Δ
...penTelemetry/Trace/BatchExportActivityProcessor.cs	37.20% <58.33%> (ø)
...mentation.Http/HttpClientInstrumentationOptions.cs	95.83% <0.00%> (-4.17%)	⬇️
...nTelemetry/Internal/OpenTelemetrySdkEventSource.cs	77.77% <0.00%> (-3.18%)	⬇️
...emetry.Api/Internal/OpenTelemetryApiEventSource.cs	61.76% <0.00%> (-2.95%)	⬇️
...us/Implementation/PrometheusExporterEventSource.cs	72.72% <0.00%> (+9.09%)	⬆️