Add cluster-aware quota limits for concurrent long-running requests

[MichaelSnowden]

What changed?
I added some cluster-aware limits on the number of concurrent long-running requests and namespace-replication-inducing requests. I also did a lot of refactoring and added a bunch of documentation around here to make this clearer because the "namespace count" terminology was not very descriptive.

Why?
These limits are designed to protect database resources, so if we just use the per-instance limits, we could end up with a total limit which is very high when there are many instances, or, conversely, too low if there are few instances.

How did you test it?
I added unit tests.

Potential risks

Is hotfix candidate?

[yux0]

What is the difference between GlobalNamespaceReplicationInducingAPIsRPS and GlobalNamespaceNamespaceReplicationInducingAPIsRPS?

[MichaelSnowden]

What is the difference between GlobalNamespaceReplicationInducingAPIsRPS and GlobalNamespaceNamespaceReplicationInducingAPIsRPS?

This was a mistake. There should only be one. I left the old one and removed the new one

[yycptt]

I don't think ns count limit is for protecting DB resource. I am under the impression that those limit exists to protect resources like network connections, file descriptors on a host. But please correct me if I am wrong.

[MichaelSnowden]

I don't think ns count limit is for protecting DB resource. I am under the impression that those limit exists to protect resources like network connections, file descriptors on a host. But please correct me if I am wrong.

This makes sense to me. I'm going to close this

[MichaelSnowden]

I'm keeping the refactoring changes though and putting them in a separate PR here: #4665