feat: measure latency

include/spore/core/ClusterManager.h

@@ -43,5 +43,6 @@ private:
 	void onHeartbeat(const char* msg);
 	void onResponse(const char* msg);
 	void onNodeInfo(const char* msg);
+	unsigned long lastHeartbeatSentAt = 0;
 	std::vector<MessageHandler> messageHandlers;
 };

include/spore/types/NodeInfo.h

@@ -17,7 +17,7 @@ struct NodeInfo {
         uint32_t cpuFreqMHz = 0;
         uint32_t flashChipSize = 0;
     } resources;
-    unsigned long latency = 0; // ms since lastSeen
+    unsigned long latency = 0; // ms from heartbeat broadcast to NODE_INFO receipt
     std::vector<EndpointInfo> endpoints; // List of registered endpoints
     std::map<String, String> labels; // Arbitrary node labels (key -> value)
 };