feat(cli): add p2p diagnostics to ping (#14426)

First PR to address #14244.

Adds common potential reasons as to why a direct connection to the workspace agent couldn't be established to `coder ping`:
- If the Coder deployment administrator has blocked direction connections (`CODER_BLOCK_DIRECT`).
- If the client has no STUN servers within it's DERP map.
- If the client or agent appears to be behind a hard NAT, as per Tailscale `netInfo.MappingVariesByDestIP`

Also adds a warning if the client or agent has a network interface below the 'safe' MTU for tailnet. This warning is always displayed at the end of a `coder ping`.

agent/api.go

@@ -37,6 +37,7 @@ func (a *agent) apiHandler() http.Handler {
 	}
 	promHandler := PrometheusMetricsHandler(a.prometheusRegistry, a.logger)
 	r.Get("/api/v0/listening-ports", lp.handler)
+	r.Get("/api/v0/netcheck", a.HandleNetcheck)
 	r.Get("/debug/logs", a.HandleHTTPDebugLogs)
 	r.Get("/debug/magicsock", a.HandleHTTPDebugMagicsock)
 	r.Get("/debug/magicsock/debug-logging/{state}", a.HandleHTTPMagicsockDebugLoggingState)

agent/health.go

@@ -0,0 +1,31 @@
+package agent
+
+import (
+	"net/http"
+
+	"github.com/coder/coder/v2/coderd/healthcheck/health"
+	"github.com/coder/coder/v2/coderd/httpapi"
+	"github.com/coder/coder/v2/codersdk"
+	"github.com/coder/coder/v2/codersdk/healthsdk"
+)
+
+func (a *agent) HandleNetcheck(rw http.ResponseWriter, r *http.Request) {
+	ni := a.TailnetConn().GetNetInfo()
+
+	ifReport, err := healthsdk.RunInterfacesReport()
+	if err != nil {
+		httpapi.Write(r.Context(), rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Failed to run interfaces report",
+			Detail:  err.Error(),
+		})
+		return
+	}
+
+	httpapi.Write(r.Context(), rw, http.StatusOK, healthsdk.AgentNetcheckReport{
+		BaseReport: healthsdk.BaseReport{
+			Severity: health.SeverityOK,
+		},
+		NetInfo:    ni,
+		Interfaces: ifReport,
+	})
+}