Include tags in trace data.

The tags item in the trace data is arbitrary metadata added to `build`
steps by Soong (and my Kati via `.KATI_TAGS` target-specific
variables). Include this in the perfetto trace so we can analyze it.

Bug: http://b/259130368
Test: End to end test tracing cp time in dist targets
Change-Id: I85d33f579dc40dbae616b24cd4cb150d86262470
diff --git a/ui/status/status.go b/ui/status/status.go
index a5b4a28..f3e58b6 100644
--- a/ui/status/status.go
+++ b/ui/status/status.go
@@ -85,6 +85,8 @@
 
 	// Involuntary context switches
 	InvoluntaryContextSwitches uint64
+
+	Tags string
 }
 
 // Counts describes the number of actions in each state