Add developer API, Prometheus metrics, and community links

lib/prometheus-response.ts

@@ -0,0 +1,48 @@
+import { createHash } from "node:crypto";
+import { formatMetrics } from "@/shared/prometheus";
+
+export async function metricsResponse(
+  request: Pick<Request, "headers">,
+  read: (tokenHash: string) => Promise<unknown>,
+) {
+  const headers = {
+    "Cache-Control": "private, no-store",
+    "X-Content-Type-Options": "nosniff",
+  };
+  const match = /^Bearer (mg_metrics_[a-f0-9]{64})$/.exec(
+    request.headers.get("authorization") ?? "",
+  );
+  if (!match)
+    return new Response("Metrics authentication required\n", {
+      status: 401,
+      headers: { ...headers, "WWW-Authenticate": "Bearer" },
+    });
+  try {
+    const start = performance.now();
+    const snapshot = await read(
+      createHash("sha256").update(match[1]).digest("hex"),
+    );
+    if (snapshot === null)
+      return new Response("Metrics authentication required\n", {
+        status: 401,
+        headers: { ...headers, "WWW-Authenticate": "Bearer" },
+      });
+    const body =
+      formatMetrics(snapshot) +
+      "# HELP minegeo_exporter_read_duration_seconds Time to read and validate the database snapshot.\n" +
+      "# TYPE minegeo_exporter_read_duration_seconds gauge\n" +
+      `minegeo_exporter_read_duration_seconds ${(performance.now() - start) / 1000}\n`;
+    return new Response(body, {
+      headers: {
+        ...headers,
+        "Content-Type": "text/plain; version=0.0.4; charset=utf-8",
+      },
+    });
+  } catch {
+    // Never return database errors, credentials, or a misleading all-zero snapshot.
+    return new Response("Metrics temporarily unavailable\n", {
+      status: 503,
+      headers,
+    });
+  }
+}