PMM-13082 Backwards compatibility with PMM v2

api/agent/pb/agent.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+package agent;
+
+import "agent/v1/agent.proto";
+
+option go_package = "api/agentpb;agentpb";
+
+// Agent service provides private methods for pmm-agent <-> pmm-managed interactions.
+service Agent {
+  // Connect establishes two-way communication channel between pmm-agent and pmm-managed.
+  rpc Connect(stream agent.v1.AgentMessage) returns (stream agent.v1.ServerMessage);
+}