Throttler multi-metrics: proto changes

[shlomi-noach]

Replaced by vitessio#16040

Description

This PR is the first in the series of incremental changes breaking down vitessio#15988 into smaller parts.

We begin by applying the proto changes.

Notable:

• The CheckThrottler gRPC call already exists in v19 and before, and is/was used by the Primary throttler to probe its replicas, and by the replica throttler to notify the Primary about recent checks.
• Here, we enhance CheckThrottlerRequest and CheckThrottlerResponse with more fields. We will later make CheckThrottler a vtctidlclient command. A check requests identifies by app name, and now also clearly indicates whether multi-metrics are supported/desired (important for backwards compatibility and cross-version interaction), as well as other flags that will be come clearer later on.
• GetThrottlerStatus, GetThrottlerStatusRequest, GetThrottlerStatusResponse are new calls, and useful for debugging/incident analysis. These return the equivalent of /throttler/status, but now via gRPC instead of HTTP. Likewise, this will be implemented as a vtctldclient in a followup PR.
• ThrottlerConfig now supports:
  • AppCheckedMetrics - mapping of app->metrics (meaning which metrics the throttler checks on behalf of given app)
  • MetricThresholds - per metric threshold (different threshold for lag, for threads_running, etc.)

See the docs in vitessio#15988 main comment for more.

This PR merges into planetscale:throttler-multi-metrics-incremental, which is a Draft PR, and not into main.

https://github.com/planetscale/vitess/compare/planetscale:vitess:throttler-multi-metrics-incremental...planetscale:vitess:throttler-multi-metrics-incremental-proto?expand=1

Related Issue(s)

vitessio#15988

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[shlomi-noach]

Replaced by vitessio#16040, whose base is vitessio:vitess:throttler-multi-metrics-incremental.