Add counter for occurrences of trailing decimal points

To help determine whether it's safe to fix the erroneous behavior.

PiperOrigin-RevId: 936003838

quiche/common/structured_headers.cc

diff --git a/quiche/common/structured_headers.cc b/quiche/common/structured_headers.cc
index ce1c82a..0b3947d 100644
--- a/quiche/common/structured_headers.cc
+++ b/quiche/common/structured_headers.cc
@@ -396,10 +396,16 @@
         LogParseError("ReadNumber", "too many digits after decimal");
         return std::nullopt;
       }
+      // TODO(b/517189418): This is never reached due to an off-by-one error.
       if (i == decimal_position) {
         LogParseError("ReadNumber", "no digits after decimal");
         return std::nullopt;
       }
+      // 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);
+      }
     }
     std::string output_number_string(input_.substr(0, i));
     input_.remove_prefix(i);