Track active projects in metrics

An active project is defined as a project where there has been at
least a buffer edit, a join request/response, or a follow update
in the last minute.

crates/rpc/src/proto.rs

@@ -134,6 +134,7 @@ messages!(
     (Ping, Foreground),
     (ProjectUnshared, Foreground),
     (RegisterProject, Foreground),
+    (RegisterProjectActivity, Foreground),
     (ReloadBuffers, Foreground),
     (ReloadBuffersResponse, Foreground),
     (RemoveProjectCollaborator, Foreground),
@@ -236,6 +237,7 @@ entity_messages!(
     PerformRename,
     PrepareRename,
     ProjectUnshared,
+    RegisterProjectActivity,
     ReloadBuffers,
     RemoveProjectCollaborator,
     RenameProjectEntry,