Documentation: Capacity Planning - states nonexistant metric 'prometheus_tsdb_head_series'

[schenklklopfer]

Describe the bug
The Documentation under https://cortexmetrics.io/docs/guides/capacity-planning/ states a metric 'prometheus_tsdb_head_series' that does not exist at all!

To Reproduce
Try to find prometheus_tsdb_head_series in /metrics on your cortex.

Expected behavior
It exists on /metrics

Environment:
Docker in single-binary mode

Additional Context
There is a closed PR that tried to add this metric to the system, but the PR was never merged: #4308

There is a comment on this PR, that states, an alternative to the given metric: #4308 (comment)

So maybe the Documentation can get fixed to that?

Instead of mentioning 'prometheus_tsdb_head_series' it could say 'cortex_ingester_memory_series_created_total - cortex_ingester_memory_series_removed_total' - if this query is still correct?

[friedrichg]

the key point in the documentation: If you have Prometheus already
This is about migrating current workloads running in Prometheus to cortex.

if you need a head series equivalent the correct value is

          sum(
            cortex_ingester_memory_series_created_total
            -
            cortex_ingester_memory_series_removed_total
          ) by (user) / 3 

Remember the replica. Yes I can try fixing the docs to also include this metric