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

ui/status/ninja_frontend/frontend.proto

@@ -79,6 +79,9 @@ message Status {
     optional uint64 voluntary_context_switches = 12;
     // Involuntary context switches
     optional uint64 involuntary_context_switches = 13;
+    // Arbitrary tags for build system profiling (module names and types, rule
+    // names, etc). Format of the string is implementation defined.
+    optional string tags = 14;
   }
 
   message Message {