Replace counter with boolean histogram for trailing decimal points

This makes it possible to measure the ratio of accepted decimal values
with and without the erroneous format, making it easier to determine
whether that behavior can be removed.

PiperOrigin-RevId: 936064965

quiche/common/structured_headers.cc

diff --git a/quiche/common/structured_headers.cc b/quiche/common/structured_headers.cc
index 0b3947d..5a3e3f4 100644
--- a/quiche/common/structured_headers.cc
+++ b/quiche/common/structured_headers.cc
@@ -21,6 +21,7 @@
 #include "absl/strings/str_format.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/span.h"
+#include "quiche/common/platform/api/quiche_client_stats.h"
 #include "quiche/common/platform/api/quiche_flag_utils.h"
 #include "quiche/common/platform/api/quiche_logging.h"
 
@@ -403,9 +404,11 @@
       }
       // Counter to track instances of b/517189418 in which trailing decimal
       // points are erroneously accepted.
-      if (i == decimal_position + 1) {
-        QUICHE_CODE_COUNT(decimal_with_zero_fractional_digits);
-      }
+      QUICHE_CLIENT_HISTOGRAM_BOOL(
+          "StructuredHeaders.DecimalWithZeroFractionalDigits",
+          i == decimal_position + 1,
+          "Whether a decimal point is erroneously accepted without any "
+          "digits following it.");
     }
     std::string output_number_string(input_.substr(0, i));
     input_.remove_prefix(i);