feat(monitor): add display name support for health monitor logging

Add a DisplayNameKey struct to pass display names from routes through the task
parent hierarchy to the health monitor. This allows the health monitor to use
more descriptive names for logging instead of internal task names.

BREAKING CHANGE: The monitor.DisplayNameKey struct is now part of the public API

internal/route/stream.go

@@ -47,6 +47,7 @@ func (r *StreamRoute) Start(parent task.Parent) error {
 	r.stream = stream
 
 	r.task = parent.Subtask("stream."+r.Name(), !r.ShouldExclude())
+	r.task.SetValue(monitor.DisplayNameKey{}, r.DisplayName())
 
 	switch {
 	case r.UseIdleWatcher():