release-22.2: roachtest: introduce grafana-dump, --skip-init

[RaduBerinde]

Backport 3/6 commits from #89191.

/cc @cockroachdb/release

Release justification: test only change, keep roachtest in sync.

Epic: none
Release note: None

---

roachprod: skip login step for roachprod grafana-start

Release note: None

roachtest: introduce a grafana-dump subcommand

This lets one create data dumps off of roachprod clusters that have been
collecting data using roachprod grafana-start. To use:

  roachprod grafana-dump $CLUSTER --dump-dir ./
  ./prometheus-docker-run.sh

The latter sets up a prometheus server on localhost:9090. To point your
grafana server to it, you can:

  brew install grafana
  brew services start grafana
  # navigate to localhost:3000

roachtest: introduce a --skip-init flag

For some tests to do have expensive initialization steps (large imports
for ex.), when iterating on them it's often useful to recycle roachtest
created clusters without re-running the full gamut. Introduce this flag
to let test authors define such fast paths.

Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is