docs: Prometheus metrics + generator (#5179)

* docs: Prometheus metrics

* Fix

* Typo

* Typo

* Typo

* Fix: link

* Update docs/admin/prometheus.md

Co-authored-by: Dean Sheather <dean@deansheather.com>

* Update docs/admin/prometheus.md

Co-authored-by: Dean Sheather <dean@deansheather.com>

* Update docs/admin/prometheus.md

Co-authored-by: Dean Sheather <dean@deansheather.com>

* Update docs/admin/prometheus.md

Co-authored-by: Dean Sheather <dean@deansheather.com>

* Update docs/admin/prometheus.md

Co-authored-by: Dean Sheather <dean@deansheather.com>

* Rephrase

* notice

* use ```shell

* Generator

* gosec

* fix: lint

* PR comments

* not needed anymore

Co-authored-by: Dean Sheather <dean@deansheather.com>
Co-authored-by: Geoffrey Huntley <ghuntley@ghuntley.com>

docs/images/icons/speed.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px" fill="#000000"><path d="M0 0h24v24H0z" fill="none"/><path d="M20.38 8.57l-1.23 1.85a8 8 0 0 1-.22 7.58H5.07A8 8 0 0 1 15.58 6.85l1.85-1.23A10 10 0 0 0 3.35 19a2 2 0 0 0 1.72 1h13.85a2 2 0 0 0 1.74-1 10 10 0 0 0-.27-10.44z"/><path d="M10.59 15.41a2 2 0 0 0 2.83 0l5.66-8.49-8.49 5.66a2 2 0 0 0 0 2.83z"/></svg>