Fix .ninja parse time reported by `-d stats`.

Because Parser::Load() is called recursively during Ninja
manifest parsing, the call to METRIC_RECORD() in this function
used to over-count the total parsing time (for example, by a
factor of 2 for the Fuchsia build).

This fixes the problem by introducing a new RECORD_METRIC_IF()
macro, which only records anything if a given condition is true.
This ensures that metric collection only starts and stops with
the outer Parser::Load() call, and none of its recursive
sub-calls.

The effect on the output of `-d stats`is, for a Fuchsia build
plan where `ninja -d stats nothing` takes a bit more than 5s:

BEFORE:
  metric                  count   avg (us)        total (ms)
  .ninja parse            27304   372.6           10172.2

AFTER:
  metric                  count   avg (us)        total (ms)
  .ninja parse            1       4165297.0       4165.3

Note that |count| went to 1, since there is only one top-level
Parser::Load() operation in this build. It would be more if
dyndeps files were loaded, which does not happen in this
build plan.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/parser.cc b/src/parser.cc
index 5f303c5..139a347 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -20,7 +20,10 @@
 using namespace std;
 
 bool Parser::Load(const string& filename, string* err, Lexer* parent) {
-  METRIC_RECORD(".ninja parse");
+  // If |parent| is not NULL, metrics collection has been started by a parent
+  // Parser::Load() in our call stack. Do not start a new one here to avoid
+  // over-counting parsing times.
+  METRIC_RECORD_IF(".ninja parse", parent == NULL);
   string contents;
   string read_err;
   if (file_reader_->ReadFile(filename, &contents, &read_err) !=