docs: document recent monitoring changes

[bobheadxi]

Adds admin and changelog documentation on some recent monitoring changes:

• https://github.com/sourcegraph/sourcegraph/issues/17072
• https://github.com/sourcegraph/sourcegraph/issues/17069
• https://github.com/sourcegraph/sourcegraph/issues/17158
• https://github.com/sourcegraph/sourcegraph/issues/17070

Dev docs also show up before admin docs if you search for "grafana" or "prometheus", so I added notices that point back to the admin docs.

[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff 4428dfd...a58cb9f.

Notify	File(s)
@christinaforney	doc/admin/observability/metrics.md doc/dev/background-information/observability/grafana.md doc/dev/background-information/observability/monitoring-generator.md doc/dev/background-information/observability/prometheus.md

[codecov]

Codecov Report

Merging #17233 (a58cb9f) into main (4428dfd) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #17233      +/-   ##
==========================================
- Coverage   52.15%   52.14%   -0.01%     
==========================================
  Files        1712     1712              
  Lines       85398    85398              
  Branches     7548     7539       -9     
==========================================
- Hits        44539    44535       -4     
- Misses      36953    36956       +3     
- Partials     3906     3907       +1     

Flag	Coverage Δ
go	51.22% <ø> (-0.01%)	⬇️
integration	30.59% <ø> (+0.02%)	⬆️
storybook	30.23% <ø> (ø)
typescript	54.39% <ø> (-0.01%)	⬇️
unit	34.87% <ø> (ø)

Impacted Files	Coverage Δ
client/web/src/tree/TreeRoot.tsx	83.33% <0.00%> (-3.04%)	⬇️
.../internal/codeintel/resolvers/graphql/locations.go	83.50% <0.00%> (-2.07%)	⬇️

[bobheadxi]

Merging to document before branch cut