[obs] add initializer metrics row to content-service dashboard

[kylos101]

Is your feature request related to a problem? Please describe

We do not know how long our different content initializers take to run

Describe the behaviour you'd like

We added two new metrics:

1. initializer_bytes_second
2. supervisor_initializer_bytes_second

Add a new Initializer metrics row to the dashboard, so that we can observe each initializer type in a separate pane to see the related P50 and P95.

note: supervisor_initializer_bytes_second is only used when we use workspacekit to restore content, which is only done when PVC feature is enabled, which is not being done at this time. initializer_bytes_second is the priority, treat supervisor_initializer_bytes_second as secondary.

Describe alternatives you've considered

Additional context

The underlying metrics were added in #15063.

@Furisto for 👀