In internal support ticket APMS-6017, a customer has an old (Java) tracer that's producing trace_ids that are hexadecimal strings.

When the C++ tracer is parsing a trace_id from a header, it accepts any string that begins with decimal digits (possibly with leading whitespace), i.e. any string matching \s*[+]?(0|[1-9][0-9]+)([^0-9].*)?. This is due to the use of std::stoull.

In this pull request, I modify the parsing code to further require that the parsed decimal integer is followed by whitespace only. This way, decimal prefixes of hexadecimal trace_ids will fail to parse, rather than successfully parsing the decimal prefix.

These changes might break things. Let's discuss if/how to roll this out.