Description of problem:
=======================
As per the discussion in https://bugzilla.redhat.com/show_bug.cgi?id=2192594#c10 and https://bugzilla.redhat.com/show_bug.cgi?id=2192594#c11, opening this bug to track the fixes to be added for retention.size.


Version-Release number of selected component (if applicable):
=============================================================
RHCS 5.3z3 


How reproducible:
=================
Always


Steps to Reproduce:
-==================
1) Deploy RHCS 5.3z3 cluster
2) Check if the Prometheus module is enabled
3) Check the existing values for flags "storage.tsdb.retention.size" and "storage.tsdb.retention.time" through API and CLI
4) Perform below 2 scenarios:

Scenario(A) :=>
* The existing values of "storage.tsdb.retention.size" and "storage.tsdb.retention.time" is 0 and 15d (default values
* Follow the KCS article https://access.redhat.com/solutions/6969242 as mentioned in #comment12 and edit the /var/lib/ceph/23adb48e-934b-11ed-8ca5-fa163e164f7f/prometheus.ceph-saya-bz-cgpcfb-node2/unit.run file to change the values as per requirement, in this test scenario, it was changed as "storage.tsdb.retention.size":"50MiB" and "storage.tsdb.retention.time":"30d".

Scenario(B) :=>
* Change the existing spec file for Prometheus daemon as mentioned in the provided doc text as follows:

# cat <<EOF | ceph orch apply -i -
service_type: prometheus
service_name: prometheus
placement:
  count: 1
spec:
  retention_time: "1y"
  retention_size: "1GB"
EOF
Scheduled prometheus update...


Actual results:
===============
The fixes added in RHCS 5.3z3 targeted only retention.time and there was a failure when tried to change the retention.size

[ceph: root@ceph-saraut-reten-n9m45s-node1-installer /]# cat <<EOF | ceph orch apply -i -
> service_type: prometheus
> service_name: prometheus
> placement:
>   count: 1
> spec:
>   retention_time: "1y"
>   retention_size: "1GB"
> EOF
Error EINVAL: ServiceSpec: __init__() got an unexpected keyword argument 'retention_size'


Expected results:
================
User should be able to modify the retention.size as well