Coordinates: allow configuring the update rate

[ShimmerGlass]

Allow to tune the coordinate update rate to reduce the load on the
servers for large datacenters.

[pierresouchay]

@Aestek this PR is weird (a bit empty)

-- EDITED -- sorry, I read badly the review, sound good to me

[slackpad]

This is auto scaled based on the size of the cluster over in https://github.com/hashicorp/consul/blob/master/agent/agent.go#L1710 and batched on the server in https://github.com/hashicorp/consul/blob/master/agent/consul/coordinate_endpoint.go#L55-L153, so we tried hard to not need to tune this :-) Were you seeing a pathological heavy write case from coordinates?

[pierresouchay]

@slackpad We are trying to limit as much the writes on our large clusters (6k+ nodes) since the leader has much to do and using 100% of CPU sometimes for long duration, we we are trying to tune not used features (such as coordinates we don't really use for now)

[slackpad]

I see. I think you can just turn off coordinates if you aren't using them:

https://github.com/hashicorp/consul/blob/master/agent/config/config.go#L198

[pierresouchay]

I see. I think you can just turn off coordinates if you aren't using them:

https://github.com/hashicorp/consul/blob/master/agent/config/config.go#L198

@slackpad Yes, you're might be right, it might be simpler

@Aestek we might abandon this one in that case

[pierresouchay]

@slackpad this one is probably not a big deal, we can live without it especially with DisableCoordinates, but #5873 might be interesting to reduce a bit the load on leader server