Drop dependency on github.com/cyberdelia/go-metrics-graphite

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

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

stats.go

@@ -13,7 +13,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	graphite "github.com/cyberdelia/go-metrics-graphite"
 	mp "github.com/nbrownus/go-metrics-prometheus"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
@@ -253,7 +252,7 @@ func (s *statsServer) buildRuntime(cfg statsConfig) ([]func(), *http.Server) {
 		// loadStatsConfig already resolved and validated the address; re-parse
 		// the resolved form (no DNS lookup) to get a *net.TCPAddr.
 		addr, _ := net.ResolveTCPAddr(cfg.graphite.protocol, cfg.graphite.resolvedAddr)
-		gcfg := graphite.Config{
+		gcfg := graphiteConfigExport{
 			Addr:          addr,
 			Registry:      metrics.DefaultRegistry,
 			FlushInterval: cfg.interval,
@@ -262,7 +261,7 @@ func (s *statsServer) buildRuntime(cfg statsConfig) ([]func(), *http.Server) {
 			Percentiles:   []float64{0.5, 0.75, 0.95, 0.99, 0.999},
 		}
 		captureFns = append(captureFns, func() {
-			if err := graphite.Once(gcfg); err != nil {
+			if err := graphiteOnce(gcfg); err != nil {
 				s.l.Error("Graphite export failed", "error", err)
 			}
 		})