Add guage to track total stream size in bytes

[kjnilsson]

Is your feature request related to a problem? Please describe.

Good to have.

Describe the solution you'd like

Every time a chunk is written the gauge is incremented, every time a segment is deleted by retention it is decremented by the size of the segment. It also needs to initialised by the the writer and/or replicas.

Describe alternatives you've considered

..

Additional context

Decision: to include index files in the same gauge or track this separately.