[observability] Default missing endpoint scheme to HTTPS

[dprotaso]

When testing prometheus endpoints I entered
some.domain.com:9090/some/path

And this took some.domain.com as the scheme. To avoid this landmine
if we end up with an opaque URL we prepend the 'https' scheme to
the endpoint

[dprotaso]

/assign @Cali0707 @evankanderson

[evankanderson]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, evankanderson

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[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 6 lines in your changes missing coverage. Please review.

Project coverage is 76.04%. Comparing base (8481e7e) to head (a28da6e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
observability/metrics/provider.go	88.46%	1 Missing and 2 partials ⚠️
observability/tracing/provider.go	88.88%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3200      +/-   ##
==========================================
+ Coverage   75.99%   76.04%   +0.05%     
==========================================
  Files         205      205              
  Lines       11709    11748      +39     
==========================================
+ Hits         8898     8934      +36     
  Misses       2541     2541              
- Partials      270      273       +3     

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