Use consistent pattern for namespace of extension methods

[alanwest]

This is an alternative to #1541.

I'm beginning to lean more towards leaving things alone for the most part. That is, I think the general rule of thumb that has been followed is to place extension methods in the same namespace as the type that they are extending. This is what Microsoft has followed with many of their libraries (e.g., Microsoft.Extensions.Configuration, Microsoft.Extensions.Logging, etc.).

However, there are a few places that do not follow this rule. I believe some of them are intentional exceptions to the rule, but some of them may have been unintentional.

I this PR, I've tracked down all of the public extension methods that are not contained in the same namespace as the type they are extending and changed them to be in the same namespace. There were two notable exceptions that I think are sensible exceptions to this general rule - I added a comment explaining why - please confirm the accuracy of my comments.

Questions:

• Does this seem like a good pattern to follow?
• Are there any namespace changes in this PR that anyone feels should be an exception to the pattern?

[codecov]

Codecov Report

Merging #1576 (a8967ce) into master (5995fd9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1576   +/-   ##
=======================================
  Coverage   80.73%   80.73%           
=======================================
  Files         242      242           
  Lines        6544     6544           
=======================================
  Hits         5283     5283           
  Misses       1261     1261           

Impacted Files	Coverage Δ
src/OpenTelemetry.Api/ActivityContextExtensions.cs	100.00% <ø> (ø)
src/OpenTelemetry.Api/Trace/ActivityExtensions.cs	94.11% <ø> (ø)
...porter.Console/ConsoleExporterLoggingExtensions.cs	0.00% <ø> (ø)
...orter.InMemory/InMemoryExporterHelperExtensions.cs	0.00% <ø> (ø)
...rter.InMemory/InMemoryExporterLoggingExtensions.cs	0.00% <ø> (ø)
...ter.Prometheus/PrometheusRouteBuilderExtensions.cs	100.00% <ø> (ø)
...enTelemetry/Resources/ResourceBuilderExtensions.cs	95.65% <ø> (ø)

[cijothomas]

This looks good to me. We want to add this to changelog before merge.

[cijothomas]

@reyang @CodeBlanch please share your thoughts as well.

[reyang]

LGTM.

[CodeBlanch]

I definitely agree with the approach that (in most cases) extensions should live in the namespace of whatever is being extended. Thanks @alanwest!