tracing: Add support for counter events

Add the TRACE_COUNTER macro for recording time-varying integer or
floating point counters.

Design doc: https://docs.google.com/document/d/1EWQ7KP6OlxlbU1PwJKniFa3gsYKoGyhI47afZhko2MA/

Change-Id: I86cd61fc4ba947ec3f522aadaa5fdc7c640084e9
diff --git a/BUILD b/BUILD
index c70775b..83a29a4 100644
--- a/BUILD
+++ b/BUILD
@@ -516,6 +516,7 @@
         "include/perfetto/tracing/interceptor.h",
         "include/perfetto/tracing/internal/basic_types.h",
         "include/perfetto/tracing/internal/checked_scope.h",
+        "include/perfetto/tracing/internal/compile_time_hash.h",
         "include/perfetto/tracing/internal/data_source_internal.h",
         "include/perfetto/tracing/internal/in_process_tracing_backend.h",
         "include/perfetto/tracing/internal/interceptor_trace_writer.h",