cmd/geth: actually enable metrics when passed via toml

[lightclient]

closes #28303, replaces #28101

--

When we added metrics to the config toml back in #22083, they didn't actually seem to get enabled when you start the client with the toml. That is fixed here by only starting the metrics after we have parsed the config for it.

You can verify it's working by running this snippet:

$ go run ./cmd/geth --metrics --metrics.addr 0.0.0.0 dumpconfig > conf && go run ./cmd/geth --config conf -dev 2>&1 | grep metrics
INFO [11-21|21:02:06.148] Enabling metrics collection
INFO [11-21|21:02:06.148] Enabling stand-alone metrics HTTP endpoint address=0.0.0.0:6060
INFO [11-21|21:02:06.148] Starting metrics server                  addr=http://0.0.0.0:6060/debug/metrics

[holiman]

Some additional tests

^C[user@work go-ethereum]$ go run ./cmd/geth --metrics --metrics.addr 0.0.0.0 dumpconfig > conf && go run ./cmd/geth --config conf -dev --metrics.addr 1.1.1.1 2>&1 | grep metrics
INFO [11-21|14:46:47.851] Maximum peer count                       ETH=50 total=50
INFO [11-21|14:46:47.873] Set global gas cap                       cap=50,000,000
INFO [11-21|14:46:47.873] Initializing the KZG library             backend=gokzg
INFO [11-21|14:46:51.231] Enabling metrics collection
INFO [11-21|14:46:51.231] Enabling stand-alone metrics HTTP endpoint address=1.1.1.1:6060
INFO [11-21|14:46:51.231] Starting metrics server                  addr=http://1.1.1.1:6060/debug/metrics
ERROR[11-21|14:46:51.232] Failure in running metrics server        err="listen tcp 1.1.1.1:6060: bind: cannot assign requested address"
^C

[user@work go-ethereum]$ go run ./cmd/geth --metrics --metrics.addr 0.0.0.0 dumpconfig > conf && go run ./cmd/geth --config conf -dev --metrics=false --metrics.addr 1.1.1.1 2>&1 | grep metrics
INFO [11-21|14:47:09.190] Maximum peer count                       ETH=50 total=50
INFO [11-21|14:47:09.211] Set global gas cap                       cap=50,000,000
INFO [11-21|14:47:09.211] Initializing the KZG library             backend=gokzg

👍

[holiman]

LGTM