Drop dependency on github.com/cyberdelia/go-metrics-graphite (#1832) (#1849)

The go-metrics-graphite package has been unmaintained for 10+ years,
which is a packaging and supply-chain concern for downstream
distributors (see #1831). Nebula only used its Config struct and Once()
entrypoint, so inline just those (~65 lines) into a local graphite.go,
preserving the upstream BSD-2-Clause copyright notice, and remove the
dependency.

Fixes #1831


(cherry picked from commit 28d82f7b8a)

Co-authored-by: Jay R. Wren <jay.wren@slack-corp.com>
Co-authored-by: Claude <svc-devxp-claude@slack-corp.com>

go.mod

@@ -7,7 +7,6 @@ require (
 	filippo.io/bigmod v0.1.0
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
 	github.com/armon/go-radix v1.0.0
-	github.com/cyberdelia/go-metrics-graphite v0.0.0-20161219230853-39f87cc3b432
 	github.com/flynn/noise v1.1.0
 	github.com/gaissmai/bart v0.28.0
 	github.com/gogo/protobuf v1.3.2