Add Prometheus service discovery endpoint #512

Job	Run time
ci-lint	11s
build-2k	4m 52s
build-calibnet	5m 13s
build-forest	4m 29s
build-debug	5m 12s
build-mainnet	5m 30s
gen-check	10m 20s
gofmt	12s
lint	3m 18s
mod-tidy-check	42s
setup-params	4s
test (test-itest-curio, ./itests/curio_test.go)	6m 45s
test (test-all, `go list ./... \| grep -v curio/itests`)	3m 1s
test (test-itest-harmonyDB, ./itests/harmonydb_test.go)	3m 25s
test (test-itest-alertnow, ./itests/alertnow_test.go)	1m 48s
	55m 2s

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