[release-18.0] Throttler: Use tmclient pool for CheckThrottler tabletmanager RPC

[shlomi-noach]

release-18.0 backport of #14979

Description

When the tablet throttler is enabled in a keyspace, the tablets within each shard make very frequent CheckThrottler RPC calls between themselves after moving from http to gRPC in #13514.

The initial implementation created a new gRPC connection and dialed the other tablet on each CheckThrottler RPC call. Because this RPC is made so frequently, however, this was not practical from a performance perspective (CPU and network overhead along with feature/input latency).

In this PR we instead leverage the existing tabletmanagerclient pooling, each tabletmanagerclient having its own gRPC connection, so that we re-use existing connections and avoid the overhead of constantly creating and destroying them on each RPC which caused a lot of tcp connection churn and related overhead.

Related Issue(s)

• Performance follow-up to: Tablet throttler: inter-checks via gRPC  #13514

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

[vitess-bot]

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[mattlord]

Just FYI, I'm pretty sure that you can add the backport labels after merge now and the backport PRs will get auto created.

[frouioui]

Just FYI, I'm pretty sure that you can add the backport labels after merge now and the backport PRs will get auto created.

this is not implemented yet :'(