export: Added "promtest" framework allowing tests against GCM and Prometheus; Added counter test.

[bwplotka]

This PR:

• Adds pkg/export/gcm/promtest pkg that contains "IngestionTest" framework.
• (The only change to export pkg) pkg/export: Adding a config option that allows passing credentials content instead of files.
• Adds pkg/export/gcm/export_gcm_test.go that implements a simple counter test that showcases the framework use.

This uses docker the e2e framework I maintain with some Thanos maintainers for Prometheus test and local export code for "GMP" test. The "local export" is so we can use local debugger and test EVERY export change in this repo.

Tests with Prometheus enabled, takes N samples * 5 seconds. We could take scrape interval to 1s, but it might kill our CI CPU. We can also disable Prometheus tests for CI and run it only locally.

Those tests will also NOT run for contributors as it requires secret.

In future PRs we can:

• Move to Cloud build for stronger security guarantees for our test SA.
• Add test for new logic for resets.
• Add GMP + GCM test (docker container too)
• Add tests for other types, different queries (e.g. rates) etc - sky is the limit.

[bwplotka]

Tests passed without mention about skipping anything, but I see it ran within 3s which is not realistic so assuming it was skipped - investigating.

Added Presubmit / test-export-gcm (pull_request) that runs just that test.

[bwplotka]

All should work, PTAL @TheSpiritXIII @pintohutch

[TheSpiritXIII]

Great work on this large PR! Left some initial feedback. :)

[bwplotka]

Thanks for review! Address all comments so far @TheSpiritXIII (:

[TheSpiritXIII]

@bwplotka I'm going to approve. Only nit is the Println. Please address and then you can merge. 🙂👍

[pintohutch]

This is great! We'd love a demo walkthrough of the e2e package at some point 😄

In general, this PR was a little intimidating in size. Can we try to put up smaller diffs for review?

WDYT about daisy-chaining baby PRs so we don't hinder development while lowering the review burden?

[bwplotka]

Thanks @TheSpiritXIII for review and @pintohutch for feedback.

This could be split a bit yea (e.g I could split local export AND e2e Prometheus into two), let's try that next time! The beauty of one atomic PR is that reviewer understands clearly why this complexity and e2e dependency is needed. There is also question PR author time, given chaining PRs in on GitHub is non trivial - definitely it would be good idea to document how we want to do it as a team 🤗