dhall: implement prometheus

[ggilmore]

• create implementation that mimics default deploy-sourcegraph's YAML with defaults
  • https://github.com/sourcegraph/sourcegraph/issues/17640#issuecomment-768110304
• write customizations that match ones implemented for other components (see documentation.md files - for example all components should support customizing resource requirements even if this container specifically doesn't do it) and souregraph.com's unique customizations
• write documentation.md file
• write tests

[github-actions]

Heads up @pecigonzalo - the "team/distribution" label was applied to this issue.

[bobheadxi]

Note: this should ideally support namespace-based generation of the configmap, see https://github.com/sourcegraph/sourcegraph/issues/17365 and sourcegraph/deploy-sourcegraph#1644

[github-actions]

Heads up @davejrt @ggilmore @dan-mckean @caugustus-sourcegraph @StephanX - the "team/delivery" label was applied to this issue.