feat(otlp): Allow to provide http client wrapped in Arc for user

[DoumanAsh]

Fixes #3467

Changes

This allows user to provide Arc wrapped client directly.
I initially thought it would be better to allow HttpClient to be Clone-able, but seeing as internally it is wrapped into Arc, I decided to allow user to do it himself so that it is possible for user to re-use this client when setting up multiple exporters

Merge requirement checklist

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

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.6%. Comparing base (146376f) to head (fce0ada).

Files with missing lines	Patch %	Lines
opentelemetry-zipkin/src/exporter/mod.rs	0.0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3468   +/-   ##
=====================================
  Coverage   83.5%   83.6%           
=====================================
  Files        127     127           
  Lines      25241   25255   +14     
=====================================
+ Hits       21091   21123   +32     
+ Misses      4150    4132   -18     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[scottgerring]

Hey @DoumanAsh thanks for raising this; it is a nice improvement to be able to re-use a HTTP client.

Could you:

• add an entry in the changelog in opentelemetry-otlp
• add a brief test that shows that the Arc<MyClient> is accepted, as well as MyClient

This is technically a breaking change, but I had a hunt for other impls of WithHttpClient external to this project and came up empty handed.

I note that zipkin has a similar-shaped with_http_client too but I think small PRs are the goal so we should leave it be.

[scottgerring]

I think it would be good to clarify that this can now take an Arc (or not), allowing the user to re-use HTTP clients if they so desire

[DoumanAsh]

I updated documentation adding doc test that illustrates both usages (which also serves as way to verify it is not breaking change)

[DoumanAsh]

Note that since I didn't want to break interface, it would require Arc with concrete HttpClient

Having Arc<dyn HttpClient> would be better, but it would not be compatible with just passing HttpClient instance

[DoumanAsh]

It seems codecov doesn't count coverage from doc tests, so I guess I can create just unit test for it