trace_processor: add template substitution support for SQL metrics

Also implement the basic process counter metrics to prove the concept.

Context: go/perfetto-metrics
Bug: 129747127
Change-Id: Ia9304328f6368bca085135fac0178243ffc0d9df
diff --git a/BUILD.gn b/BUILD.gn
index 084bf9d..830a488 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -138,7 +138,10 @@
     deps += [ "src/profiling/memory:unittests" ]
   }
   if (perfetto_build_standalone && !is_android) {
-    deps += [ "src/trace_processor:unittests" ]
+    deps += [
+      "src/trace_processor:unittests",
+      "src/trace_processor/metrics:unittests",
+    ]
   }
 }