[prometheus] Change storage.retention schema to allow for storage.tsdb.retention.size to be configured

[RohitKochhar]

Is your feature request related to a problem ?

Currently, to set the storage.tsdb.retention.size, it must be handed to the chart via the extraFlags command, by setting storage.tsdb.retention.size=XGB.

Since Helm doesn't support appending to lists of default values, this causes the default extraFlags arguments to be overwritten, requiring them to be redefined explicitly.

Describe the solution you'd like.

I would like to change the schema of the server.retention variable so that the desired retention size could be configured by setting it as server.retention.size=XGB in values.yaml, which would then pass the desired configuration as a flag to the Prometheus server (similar to the behaviour of server.retention, which passes the desired retention time as a flag ,storage.tsdb.retention.time, to the server.)

Describe alternatives you've considered.

I have tried passing the retention size to the server using extraArgs, extraFlags and defaultFlagsOverride, but all cause the default values to be overwritten.

Additional context.

No response