a8n: Load and store Bitbucket Server PR activity as ChangesetEvents

Created by: tsenart

Part of #6085

Created by: codecov[bot]

Codecov Report

Merging #6339 into master will increase coverage by 0.05%. The diff coverage is 28%.

@@            Coverage Diff             @@
##           master    #6339      +/-   ##
==========================================
+ Coverage   39.19%   39.24%   +0.05%     
==========================================
  Files        1188     1187       -1     
  Lines       60858    60856       -2     
  Branches     5813     5808       -5     
==========================================
+ Hits        23853    23885      +32     
+ Misses      34815    34778      -37     
- Partials     2190     2193       +3

Impacted Files	Coverage Δ
internal/extsvc/bitbucketserver/client.go	44.61% <0%> (-0.14%)
internal/a8n/types.go	21.12% <27.9%> (+7.61%)
cmd/repo-updater/repos/bitbucketserver.go	80.66% <33.33%> (-0.8%)
enterprise/pkg/a8n/store.go	84.15% <33.33%> (+1.89%)
shared/src/backend/fetch.ts	0% <0%> (-25%)
internal/search/backend/horizontal.go	88.7% <0%> (-1.62%)
browser/src/platform/context.ts	32.55% <0%> (-1.59%)
browser/src/libs/code_intelligence/inject.ts	0% <0%> (ø)
browser/src/libs/code_intelligence/extensions.tsx	82.43% <0%> (ø)
browser/src/shared/backend/requestGraphQL.ts
... and 4 more

Created by: tsenart

Question: do you think we should throttle the ChangesetSyncer now that we make more requests to BBS? Or would adding a warning to the Changelog be enough?

Given that we intend to support a scale of only ~200 repos in campaigns for now, I think a CHANGELOG entry would be enough. That doesn't eliminate the need for smarter syncing in the future (when we need to support larger scales), though. Even when we have webhooks, the syncing logic must be much smarter than re-fetching everything every single time it runs.

I realise we have been talking about this problem for a while but haven't captured it in an issue, so here it goes: https://gitlab.sgdev.org/root/sourcegraph/-/issues/6388

Created by: tsenart

Added a TODO to our 3.10 tracking issue:

Update CHANGELOG to mention additional requests done by repo-updater to sync A8N data and to warn about its current limits of scalability / potential impact on code host when used beyond a certain scale.