cmd/geth: allow configuring metrics HTTP server on separate endpoint (#21290)

Exposing /debug/metrics and /debug/metrics/prometheus was dependent
on --pprof, which also exposes other HTTP APIs. This change makes it possible
to run the metrics server on an independent endpoint without enabling pprof.
# Conflicts:
#	cmd/utils/flags.go
#	internal/debug/flags.go
#	metrics/exp/exp.go
#	mobile/geth.go