metrics: add a basic setup, collect metrics for the server

metrics/dashboards/README.md

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+# quic-go Prometheus / Grafana setup
+
+Expose a Grafana endpoint on `http://localhost:5001/prometheus`:
+```go
+import "github.com/prometheus/client_golang/prometheus/promhttp"
+
+go func() {
+    http.Handle("/prometheus", promhttp.Handler())
+    log.Fatal(http.ListenAndServe(":5001", nil))
+}()
+```
+
+Set a metrics tracer on the `Transport`:
+```go
+quic.Transport{
+	Tracer: metrics.NewTracer(),
+}
+```
+
+When using multiple `Transport`s, it is recommended to use the metrics tracer struct for all of them.
+
+Running:
+```shell
+docker-compose up
+```

metrics/dashboards/datasources.yml

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+apiVersion: 1
+
+deleteDatasources:
+  - name: Prometheus
+    orgId: 1
+
+datasources:
+  - name: Prometheus
+    orgId: 1
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    editable: false

metrics/dashboards/docker-compose.yml

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+version: '3.8'
+
+volumes:
+  prometheus_data: {}
+  grafana_data: {}
+
+services:
+  prometheus:
+    image: prom/prometheus:latest
+    container_name: prometheus
+    volumes:
+      - ./prometheus.yml:/etc/prometheus/prometheus.yml
+      - prometheus_data:/prometheus
+    command:
+      - '--config.file=/etc/prometheus/prometheus.yml'
+    expose:
+      - 9090
+  grafana:
+    image: grafana/grafana:latest
+    container_name: grafana
+    volumes:
+      - grafana_data:/var/lib/grafana
+      - ./datasources.yml:/etc/grafana/provisioning/datasources/prom.yml
+    ports:
+      - "3000:3000"

metrics/dashboards/prometheus.yml

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+global:
+  scrape_interval: 15s
+
+scrape_configs:
+  - job_name: 'quic-go'
+    scrape_interval: 15s
+    static_configs:
+      - targets: ['host.docker.internal:5001']
+    metrics_path: '/prometheus'