Update module github.com/prometheus/client_golang to v1.21.0 (#589)

go.mod

go.sum

....com/prometheus/client_golang/prometheus/atomic_update.go

...github.com/prometheus/client_golang/prometheus/counter.go

...or/github.com/prometheus/client_golang/prometheus/desc.go

...r/github.com/prometheus/client_golang/prometheus/gauge.go

...rometheus/client_golang/prometheus/go_collector_latest.go

...thub.com/prometheus/client_golang/prometheus/histogram.go

...m/prometheus/client_golang/prometheus/internal/difflib.go

...s/client_golang/prometheus/internal/go_runtime_metrics.go

.../github.com/prometheus/client_golang/prometheus/metric.go

.../prometheus/client_golang/prometheus/process_collector.go

...s/client_golang/prometheus/process_collector_cgo_darwin.c

.../client_golang/prometheus/process_collector_cgo_darwin.go

...heus/client_golang/prometheus/process_collector_darwin.go

...lient_golang/prometheus/process_collector_nocgo_darwin.go

...theus/client_golang/prometheus/process_collector_other.go

...s/client_golang/prometheus/process_collector_wasip1_js.go

...eus/client_golang/prometheus/process_collector_windows.go

....com/prometheus/client_golang/prometheus/promhttp/http.go