Metrics env check fix + Gitignore to grafana JSON files and link to doc

[slemrmartin]

Issue: https://github.com/RedHatInsights/topological_inventory-api/issues/320

Just a small addition to #322. Gragana dashboard changes have to be exported to JSON as frequently as possible because you never know when it will be replaced by appsre.

And JSONs are then converted to configmaps, so storing them locally in the same folder makes sense to me.

This PR also adds a link to documentation in Topological Inventory Guides.

EDIT: Changed check for ENV["METRICS_PORT"] which is not ignoring default settings

• [x] https://github.com/RedHatInsights/topological_inventory-guides/pull/36 (depends on, but doesn't need to wait for it)

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RHCLOUD-9941

[dippy-bot]

Checked commits https://github.com/slemrmartin/topological_inventory-api/compare/667386a15de8d64b843d6e0f3a196205b57a0ffc~...ae434c670b9af62792dd8f687db9f6b6c5236b4a with ruby 2.5.7, rubocop 0.69.0, haml-lint 0.35.0, and yamllint 2 files checked, 0 offenses detected Everything looks fine. :star: