controller: disable prometheus metric processor memory

[cbgbt]

Issue number:
Closes #441

Description of changes:
Per #441 and testing, the gauges reported by the controller's /metrics endpoint would often report stale data, double-counting hosts as it showed the historic state alongside the current state.

This is because with_memory() was enabled, which explicitly maintains previous metrics when only new subsets are reported.

This change disables metric processor memory to make it truly stateless, which is how we were computing the metrics anyways.

In this case, prometheus will still report old state until the metrics are stale. In order to attempt to clear old state faster, we also explicitly report 0 counts for prior metrics when we can. Because the controller is supposed to be stateless and can be rescheduled, this is done as a best-effort.

Testing done:
Watched the controller /metrics endpoint with and without this change.

After the change was made, the metrics endpoint always surfaced the current state of the cluster, e.g.:

# HELP brupop_hosts_state Brupop host's state
# TYPE brupop_hosts_state gauge
brupop_hosts_state{service_name="unknown_service",state="Idle"} 2
brupop_hosts_state{service_name="unknown_service",state="RebootedIntoUpdate"} 1
brupop_hosts_state{service_name="unknown_service",state="StagedAndPerformedUpdate"} 0
# HELP brupop_hosts_version Brupop host's bottlerocket version
# TYPE brupop_hosts_version gauge
brupop_hosts_version{bottlerocket_version="1.14.0",service_name="unknown_service"} 2
brupop_hosts_version{bottlerocket_version="1.14.2",service_name="unknown_service"} 1

(Note that StagedAndPerformedUpdate has a count of 0, because the host that is now in RebootedIntoUpdate had it's prior count intentionally wiped)

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[cbgbt]

^ Fixed a typo in the git commit message, plus rebased onto develop.