[VSA] Median data is being requested unnecessarily for hidden stages

Problem to solve

On gitlab-org, some Value Stream Analytics (VSA) stages are hidden, however the median data is still loaded.

Further details

This was reported in #225405 (comment 371403582).

Proposal

In Value Stream Analytics, do not request the median data for hidden stages. changes we need to make are in the following places:

• Stage median data
• Duration chart data
• Duration chart median

In those locations we are looping over all the stages, then fetching their data instead we should use the activeStages getter which excludes the hidden stages.