feat(relay): record the Cloudflare edge each request entered through

infra/relay/src/http/Api.test.ts

@@ -1196,6 +1196,40 @@ describe("relay request tracing", () => {
       }),
   );
 
+  it.effect("records the Cloudflare edge a request entered through on its server span", () =>
+    Effect.gen(function* () {
+      const spans: Array<Tracer.NativeSpan> = [];
+      const tracer = Tracer.make({
+        span: (options) => {
+          const span = new Tracer.NativeSpan(options);
+          spans.push(span);
+          return span;
+        },
+      });
+      const source = new Request("https://relay.test/v1/environments/env-1/connect", {
+        method: "POST",
+      });
+      Object.defineProperty(source, "cf", {
+        value: { colo: "JNB", country: "ZA", continent: "AF", asn: 37_100 },
+      });
+
+      yield* traceRelayHttpRequestWith(
+        Effect.succeed(HttpServerResponse.empty({ status: 204 })),
+        Layer.succeed(Tracer.Tracer, tracer),
+      ).pipe(
+        Effect.provideService(
+          HttpServerRequest.HttpServerRequest,
+          HttpServerRequest.fromWeb(source),
+        ),
+      );
+
+      const server = spans.find((span) => span.kind === "server");
+      expect(server?.attributes.get("relay.edge.colo")).toBe("JNB");
+      expect(server?.attributes.get("relay.edge.country")).toBe("ZA");
+      expect(server?.attributes.get("relay.edge.continent")).toBe("AF");
+    }),
+  );
+
   it.effect("records one server span inside the worker's disabled HTTP tracer", () =>
     Effect.gen(function* () {
       const spans: Array<Tracer.NativeSpan> = [];

infra/relay/src/http/Api.ts

@@ -207,6 +207,21 @@ const relayRequestDeadline = <E, R>(
 
 const SENDER_TRACE_HEADER = /^(traceparent|tracestate|b3|x-b3-.*)$/i;
 
+// Where the request entered Cloudflare's network. The Worker runs near its
+// database, not at this edge, so these are the only way to split relay
+// latency by where users are.
+const annotateRelayEdgeLocation = Effect.gen(function* () {
+  const request = yield* HttpServerRequest.HttpServerRequest;
+  const cf: unknown = request.source instanceof Request ? Reflect.get(request.source, "cf") : null;
+  if (typeof cf !== "object" || cf === null) return;
+  const attributes: Record<string, string> = {};
+  for (const key of ["colo", "country", "continent"] as const) {
+    const value: unknown = Reflect.get(cf, key);
+    if (typeof value === "string" && value) attributes[`relay.edge.${key}`] = value;
+  }
+  yield* Effect.annotateCurrentSpan(attributes);
+});
+
 // The method-named server span is created below; this wrapper must not add another span.
 export const traceRelayHttpRequest = Effect.fnUntraced(function* <E, R>(
   httpEffect: Effect.Effect<
@@ -216,7 +231,8 @@ export const traceRelayHttpRequest = Effect.fnUntraced(function* <E, R>(
   >,
 ) {
   const request = yield* HttpServerRequest.HttpServerRequest;
-  const traced = appendRelayTraceContextResponseHeader.pipe(
+  const traced = annotateRelayEdgeLocation.pipe(
+    Effect.andThen(appendRelayTraceContextResponseHeader),
     Effect.andThen(relayRequestDeadline(httpEffect)),
   );
   if (!isRelayHookPath(request.url)) {
@@ -238,7 +254,8 @@ export const traceRelayHttpRequest = Effect.fnUntraced(function* <E, R>(
     ),
   });
   return yield* HttpMiddleware.tracer(
-    appendRelayTraceContextResponseHeader.pipe(
+    annotateRelayEdgeLocation.pipe(
+      Effect.andThen(appendRelayTraceContextResponseHeader),
       Effect.andThen(
         relayRequestDeadline(
           httpEffect.pipe(Effect.provideService(HttpServerRequest.HttpServerRequest, request)),