Bump prometheus-client from 0.19.0 to 0.20.0 #1384

Job	Run time
changes	3s
check-lockfile	5s
lint-crlf	6s
lint-newsfile	0s
check-sampleconfig	2m 23s
lint	1m 30s
Typechecking	3m 8s
check-schema-delta	37s
lint-pydantic	2m 20s
lint-clippy	0s
lint-clippy-nightly	0s
lint-rustfmt	0s
linting-done	3s
calculate-test-jobs	6s
trial-olddeps	6m 57s
complement (monolith, SQLite)	12m 28s
portdb (3.8, 11)	3m 30s
trial-pypy	0s
complement (monolith, Postgres)	13m 50s
portdb (3.11, 15)	3m 30s
complement (workers, Postgres)	20m 47s
cargo-bench	0s
cargo-test	0s
trial (3.8, sqlite, all)	5m 48s
sytest (focal)	8m 6s
trial (3.8, postgres, 11, all)	11m 50s
sytest (focal, postgres)	10m 12s
trial (3.8, sqlite)	4m 35s
sytest (focal, multi-postgres, workers)	12m 50s
sytest (focal, multi-postgres, workers, asyncio)	19m 46s
export-data	3m 11s
tests-done	3s
	2h 27m 44s

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