Optimize performance when request tracing is disabled

Currently, request tracing is enabled by default, which seems to be an accidental oversight.

Additionally, even when NullRequestTracer is in use there is some unnecessary overhead building tags around string formatting. This uses extra CPU cycles and creates additional heap allocations. This could be addressed by skipping the string format step when the NullRequestTracer is in use.

Alternatively, the tag system could be redesigned to delay string formatting until the span is complete and being processed. This would have the advantage of avoiding the string format expense if the span is lower than the threshold for tracing.