Handle trailing slash on endpoint for otlphttp

[cartermp]

This makes it so we don't put a double slash on the endpoint for otlphttp when it exists.

The spec says that this kind of stuff should be handled w.r.t. the OTEL_EXPORTER_OTLP_ENDPOINT env var and says nothing about programmatic handling of this config. But I still think it's a worthwhile change on a common-ish[1] ergonomic issue.

[1] while the collector isn't the only source of this, at honeycomb we emitted an error related to a misconfigured otlphttp endpoint ~2.5 million times over the past month

fixes #8084

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3f71668) 90.70% compared to head (20cc277) 90.70%.

❗ Current head 20cc277 differs from pull request most recent head 11ca87c. Consider uploading reports for the commit 11ca87c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8083   +/-   ##
=======================================
  Coverage   90.70%   90.70%           
=======================================
  Files         300      300           
  Lines       15164    15167    +3     
=======================================
+ Hits        13754    13757    +3     
  Misses       1128     1128           
  Partials      282      282           

Impacted Files	Coverage Δ
exporter/otlphttpexporter/factory.go	87.23% <100.00%> (+0.42%)	⬆️

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[TylerHelmuth]

@cartermp please add a changelog entry by running make chlog-new.

[TylerHelmuth]

Also please add some new tests to factory_test.go to test this behavior.

[cartermp]

testing.go:1446: race detected during execution of test

did I do this or is this just kinda a thing sometimes?