Add Prometheus service discovery endpoint #510

Job	Run time
ci-lint	1m 13s
build-2k	4m 28s
build-debug	4m 16s
build-calibnet	4m 17s
build-forest	4m 22s
gen-check	9m 54s
build-mainnet	4m 13s
lint	3m 18s
gofmt	14s
mod-tidy-check	38s
setup-params	4s
test (test-itest-curio, ./itests/curio_test.go)	7m 11s
test (test-all, `go list ./... \| grep -v curio/itests`)	3m 23s
test (test-itest-harmonyDB, ./itests/harmonydb_test.go)	3m 47s
test (test-itest-alertnow, ./itests/alertnow_test.go)	2m 2s
	53m 20s

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