feat(dashboard): allow overriding the datasource

What

Allow the caller to override the datasource from the default of $PROMETHUS_DS.

Why

With the introduction of Loki as a datasource we want to have multiple panels in the same dashboard using different data sources.

For example: !6171 (merged)

Testing

• basic.graphPanel: https://dashboards.gitlab.net/dashboard/snapshot/2xWU6iojWi8jCQdlMXNuOpL10IJe2sT9?orgId=1&viewPanel=3&inspectTab=json

  $ ./test-dashboard.sh ./importers/github-importer.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/2xWU6iojWi8jCQdlMXNuOpL10IJe2sT9 - importers: Github Importer

• basic.heatmap: https://dashboards.gitlab.net/dashboard/snapshot/KLLxkXX8gIOTHFU5yL6RTTF1motx48pU?orgId=1&viewPanel=6&inspectTab=json

  $ ./test-dashboard.sh ./sentry/events.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/KLLxkXX8gIOTHFU5yL6RTTF1motx48pU - sentry: Events

• basic.table: https://dashboards.gitlab.net/dashboard/snapshot/NYpch9eKEXnJIimsgzSHwO70RyAXdH62?orgId=1&viewPanel=3&inspectTab=json

  $ ./test-dashboard.sh ./delivery/release_management_toil.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/NYpch9eKEXnJIimsgzSHwO70RyAXdH62 - delivery: Release Management Toil

• `basic.multiTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/KtV6iL9iFfWFoxPMstsRE3uiIzUn1Dau?orgId=1&viewPanel=79&inspect=79&inspectTab=json

  $ ./test-dashboard.sh ./praefect/main.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/KtV6iL9iFfWFoxPMstsRE3uiIzUn1Dau - praefect: Overview

• basic.queueLengthTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/0gWYPV3x5cZ36nr5Jx4qbNK74R47tffR?orgId=1&viewPanel=140&inspect=140&inspectTab=json

  $ ./test-dashboard.sh ./sidekiq/main.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/0gWYPV3x5cZ36nr5Jx4qbNK74R47tffR - sidekiq: Overview

• basic.percentageTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/4C0Sg8Bj2U7y9GQ4UpG4I6eV0hVonf5e?orgId=1&viewPanel=2383655193&inspect=2383655193&inspectTab=json

  $ ./test-dashboard.sh ./sidekiq/main.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/0gWYPV3x5cZ36nr5Jx4qbNK74R47tffR - sidekiq: Overview

• basic.apdexTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/4C0Sg8Bj2U7y9GQ4UpG4I6eV0hVonf5e?orgId=1&viewPanel=750569445&inspect=750569445&inspectTab=json

  $ ./test-dashboard.sh ./sidekiq/main.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/0gWYPV3x5cZ36nr5Jx4qbNK74R47tffR - sidekiq: Overview

• basic.latencyTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/NAjWWyF6okkWxhItlHO8MzsAQvX1AcMx?orgId=1&viewPanel=20&inspect=20&inspectTab=json

  $ ./test-dashboard.sh ./registry/database.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/NAjWWyF6okkWxhItlHO8MzsAQvX1AcMx - registry: Database Detail

• basic.slaTimeseries: https://dashboards.gitlab.net/dashboard/snapshot/LyUvnPJmSkdko53aUHw3IDaQGoZvj9pQ?orgId=1&viewPanel=12&inspect=12&inspectTab=json

  $ ./test-dashboard.sh ./general/slas.dashboard.jsonnet
  Installed https://dashboards.gitlab.net/dashboard/snapshot/LyUvnPJmSkdko53aUHw3IDaQGoZvj9pQ - general: SLAs

Reference: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23969