fix: Pin Prometheus and Grafana revisions #677

Job	Run time
Publish Charm / Generate the Charm Matrix	3s
Run Tests / Individual charm's integration tests (controller)	15m 20s
Run Tests / Individual charm's integration tests (ui)	15m 29s
Run Tests / Bundle Integration	25m 46s
Run Tests / Lint Code (controller)	27s
Run Tests / Lint Code (ui)	36s
Run Tests / Unit Test (controller)	47s
Run Tests / Unit Test (ui)	54s
Run Tests / Check libraries (jupyter-ui) / Check libraries	28s
Run Tests / Check libraries (jupyter-controller) / Lint	10s
Run Tests / Check libraries (jupyter-controller) / Check libraries	29s
Run Tests / Check libraries (jupyter-ui) / Lint	14s
Publish Charm / Publish Charm (./charms/jupyter-controller)	5m 40s
Publish Charm / Publish Charm (./charms/jupyter-ui)	5m 32s
	1h 11m 55s

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