### What
The heaviest requests we serve are the register and metrics POSTs from
our SDKs/clients.
This PR adds ratelimiting to /api/client/register, /api/client/metrics,
/api/frontend/register and /api/frontend/metrics with a default set to
6000 requests per minute (or 100 rps) for each of the endpoints.
It will be overrideable by the environment variables documented.
### Points of discussion
@kwasniew already suggested using featuretoggles with variants to
control the rate per clientId. I struggled to see if we could
dynamically update the middleware after initialisation, so this attempt
will need a restart of the pod to update the request limit.
