fix/repo-updater: add WARN level logs every time we sync a code host

[ggilmore]

Closes https://linear.app/sourcegraph/issue/SRC-557/sourcegraph-deleted-all-repos-during-github-outage

Github recently had an incident, that caused a number of Sourcegraph's users repos to be deleted and recloned.

After auditing the code, we believe a number / most of these cases are a form of a byzantine fault. Namely, the github API was failing in such a way that it was returning valid-looking (200 status code, well-formed JSON responses) responses that were otherwise incorrect (e.g. returning an empty list [] for the set of repositories) instead of a more easily detectable error (non-200 status code, malformed JSON, etc.)

In order to be able to determine this in the future, this PR adds more WARN level logs to the syncing logic that logs the number of repositories that were added/removed/modified/etc. + whether or not an error occurred during every sync.

We chose the WARN log level to that it will be emitted by default on all Sourcegraph deployments. This will make for more voluminous logs, but its a critical debugging tool that we need.

Test plan

Run sg start, and see repo-updater logs that look like:

[   repo-updater] WARN repo-updater.syncer repos/syncer.go:377 finished listing repositories from external service {"svc": {"name": "GITHUB #1", "id": 3}, "syncProgress": {"synced": 22, "errors": 0, "added": 22, "removed": 0, "modified": 0, "unmodified": 0}, "seen": 22, "modified": true, "error": "<nil>"}

Changelog

repo-updater now emits logs that log the result of every code host sync.

[ggilmore]

• fix/repo-updater: add WARN level logs every time we sync a code host #64519 👈
• main

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