feat: add channel field to query context (#183)

* feat: add source field to query context

* refactor: rename field to channel

* chore: fix indent

proto/greptime/v1/common.proto

@@ -25,6 +25,7 @@ message QueryContext {
   string current_schema = 2;
   string timezone = 4;
   map<string, string> extensions = 5;
+  uint32 channel = 6;
 }
 
 message RequestHeader {

proto/greptime/v1/meta/heartbeat.proto

@@ -23,7 +23,7 @@ import "greptime/v1/meta/common.proto";
 service Heartbeat {
   // Heartbeat, there may be many contents of the heartbeat, such as:
   // 1. Metadata to be registered to meta server and discoverable by other
-  // nodes.
+  //    nodes.
   // 2. Some performance metrics, such as Load, CPU usage, etc.
   // 3. The number of computing tasks being executed.
   rpc Heartbeat(stream HeartbeatRequest) returns (stream HeartbeatResponse) {}