trace_processor: add support for proto and sql metric from shell

This CL introduces support for metrics and metric protos to be added to
the trace processor from the shell. Only single files of each are
supported - this restriction may be relaxed in the future.

Bug: 129747127
Change-Id: Ia80c378bc0ed7289e85f8886c9efbe08bd749fbe
diff --git a/BUILD.gn b/BUILD.gn
index 4a2c960..846d132 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -108,6 +108,7 @@
     deps = [
       "src/trace_processor:trace_processor_shell",
     ]
+    testonly = true  # We need this for proto full.
   }
 }