output/cloudv2: Use a static remote service url

[codebien]

It moves the sets of the test run id and the relative URL generation from per request to once per client.

[codecov-commenter]

Codecov Report

Merging #3125 (44b9843) into master (629d0fd) will decrease coverage by 0.12%.
The diff coverage is 71.42%.

❗ Current head 44b9843 differs from pull request most recent head 9239f74. Consider uploading reports for the commit 9239f74 to get more accurate results

@@            Coverage Diff             @@
##           master    #3125      +/-   ##
==========================================
- Coverage   73.90%   73.79%   -0.12%     
==========================================
  Files         243      241       -2     
  Lines       18489    18483       -6     
==========================================
- Hits        13664    13639      -25     
- Misses       3958     3972      +14     
- Partials      867      872       +5     

Flag	Coverage Δ
ubuntu	73.79% <71.42%> (-0.03%)	⬇️
windows	?

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

Impacted Files	Coverage Δ
output/cloud/expv2/metrics_client.go	59.37% <50.00%> (-1.24%)	⬇️
output/cloud/expv2/flush.go	84.61% <100.00%> (ø)
output/cloud/expv2/output.go	86.84% <100.00%> (ø)

... and 12 files with indirect coverage changes

[imiric]

@codebien I'm waiting for this PR to also be rebased and the conflicts resolved before reviewing it.