staging/objstorage-rpc: Activate gunicorn statsd metrics

So [1] can be tested in next-version environment.

So far so good, it's working as expected. [2]

Further commits adds the keda autoscaler based on the new metrics and activate it on the next version environment.

The scaledobject are not triggered because their threshold is defined at 1 and currently the sum is below it [3]. If adapting the threshold to 0.1, the objstorage pods are indeed scaling.

[1] !453 (closed)

[2]

[3] https://grafana.softwareheritage.org/goto/H8FlXCrIz?orgId=1

[4]

|-------------------------+---------------------------+-------|
| Time                    | deployment                | Value |
|-------------------------+---------------------------+-------|
| 2024-08-09 10:20:00.385 | objstorage-ro-pathslicing | 0.200 |
| 2024-08-09 10:20:00.385 | objstorage-rw-pathslicing | 0.355 |
|-------------------------+---------------------------+-------|

Refs. swh/infra/sysadm-environment#1592 (closed)