Introduce heartbeats

proto/opamp.proto

@@ -264,6 +264,20 @@ message OpAMPConnectionSettings {
     // This field is optional: if omitted the client SHOULD NOT use a client-side certificate.
     // This field can be used to perform a client certificate revocation/rotation.
     TLSCertificate certificate = 3;
+
+    // The Agent MUST periodically send an AgentToServer message if this field is set. At a minimum
+    // the instance_uid field MUST be set. It is recommended that the Agent also set ComponentHealth
+    // as well.
+    //
+    // A Polling-based HTTP Client MUST use the value as polling interval.
+    //
+    // A heartbeat is used to keep a load balancer connection active AND inform the server that the Agent
+    // is still alive and active.
+    //
+    // This field is optional:
+    // if omitted, a default heartbeat interval of 30 seconds should be used.
+    // if set to zero, the heartbeat ability will be disabled.
+    optional uint64 heartbeat_interval_seconds = 4;
 }
 
 // The TelemetryConnectionSettings message is a collection of fields which comprise an