Make Telemeter receive endpoint request limit configurable

Currently, Telemeter is not equipped with configurable request limit for receive endpoint (for full context see: https://github.com/openshift/cluster-monitoring-operator/pull/1416). It is using the default limit defined in the code base, however it seems this limit might not be suitable for our usage.

As a part of this ticket, it should be:

1) Understood what is the appropriate limit for request size for our use cases

2) Make the limit configurable in Telemeter via a flag

3) Deploy the changes, initially to the staging environment, to enable our team to test it.