[Instrumentation.Http] follow up from previous pr

[TimothyMothra]

Follow up to #4870

Changes

• update changelog to mention new metrics are available for both HttpClient and HttpWebRequest
• update Readme to describe both new metrics http.client.duration and http.client.request.duration.
• update unit test.
  • remove extra IF statement.
  • assert units. (s and ms).
  • assert histogram bounds.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Merging #4919 (d8df973) into main (d4d5122) will increase coverage by 0.01%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4919      +/-   ##
==========================================
+ Coverage   83.28%   83.30%   +0.01%     
==========================================
  Files         295      295              
  Lines       12324    12324              
==========================================
+ Hits        10264    10266       +2     
+ Misses       2060     2058       -2     

Flag	Coverage Δ
unittests	83.30% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

see 4 files with indirect coverage changes

[CodeBlanch]

Seems redundant to me because everything in this library w.r.t. .NET Framework works for both HttpClient & HttpWebRequest.

opentelemetry-dotnet/src/OpenTelemetry.Instrumentation.Http/README.md

Lines 6 to 12 in dc1f09d

	This is an [Instrumentation
	Library](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/glossary.md#instrumentation-library),
	which instruments
	[System.Net.Http.HttpClient](https://docs.microsoft.com/dotnet/api/system.net.http.httpclient)
	and
	[System.Net.HttpWebRequest](https://docs.microsoft.com/dotnet/api/system.net.httpwebrequest)
	and collects metrics and traces about outgoing HTTP requests.

I would say we should only mention it if it didn't work for one or the other for some reason.

Not a blocker for me, just sayin' 🤔

[CodeBlanch]

LGTM

[utpilla]

Suggested change

	* Introduced a new metric for `HttpClient`, `http.client.request.duration`
	* Introduced a new metric for `HttpClient` named `http.client.request.duration` which is measured in seconds