fix(metricprovider): reuse http.Transport for http.Client

[kevinji]

The current code creates a new http.Transport for each http.Client that is created, which leads to a leak in TCP connections due to keep-alive.

Instead, reuse the same http.Transport between requests. According to the http.Transport docs, this is safe for concurrent use.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 24m 50s ⏱️
112 tests 100 ✅  7 💤 5 ❌
456 runs  421 ✅ 28 💤 7 ❌

For more details on these failures, see this check.

Results for commit 8373787.

[github-actions]

Published Unit Test Results

2 257 tests   2 257 ✅  2m 58s ⏱️
  128 suites      0 💤
    1 files        0 ❌

Results for commit 8373787.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.98%. Comparing base (825a7ca) to head (8373787).
Report is 65 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3780      +/-   ##
==========================================
+ Coverage   83.97%   83.98%   +0.01%     
==========================================
  Files         161      161              
  Lines       18492    18495       +3     
==========================================
+ Hits        15528    15533       +5     
+ Misses       2100     2099       -1     
+ Partials      864      863       -1     

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