processor: Make metrics an optional compile-time dep

Makes it possible to compile trace processor without metrics support,
which significantly reduces its binary size.

Bug: 141459049
Change-Id: I38812f33312b8d921d9647c09cb5a96d0adc9914
diff --git a/BUILD.gn b/BUILD.gn
index f73a5e9..be4b48c 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -39,7 +39,8 @@
   ]
 }
 
-if (enable_perfetto_trace_processor) {
+if (enable_perfetto_trace_processor &&
+    enable_perfetto_trace_processor_metrics) {
   all_targets += [ "src/trace_processor:trace_processor_shell" ]
 }