Use metric step if present

What does this MR do?

Fixes #212491 (closed)

Currently in our monitoring dashboard solution, when we provide queries that aggregate data in say one day at a time and that generates a data point, we should plot the aggregated data by the interval provided by the query.

This is because we currently calculate step https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/monitoring/stores/actions.js#L147. Grafana gets around this by automatically setting step to interval. We may do this in future iterations.

This MR allows us to use step when it is defined in a dashboard yml. The step key is automatically parsed in the backend, sent to the frontend and kept in the store, we just needed to use it.

An example dashboard yml

dashboard: general SLAs
panel_groups:
- group: Headline
  panels:
  - title: Weighted Availability Score - GitLab.com
    max_value: 1
    metrics:
    - id: single-stat-weighted-availability
      label: Weighted Availability Score - GitLab.com
      query: avg_over_time(sla:gitlab:ratio{environment="gprd", stage="main"}[3m])
      unit: "%"
      step: 10

Documentation MR: !29194 (diffs)

Screenshots

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