ddtrace/tracer.Measured(): Cache a global instance (save 1 alloc/call)

[evanj]

This saves one allocation for every call to this function. This
function is called in about 26 places in contrib/..., which includes
some fairly hot code paths, such as in the gRPC interceptors. The
gRPC interceptor microbenchmark shows this saves about ~8% CPU, and
reduces one allocation per call. This is part of my attempt to reduce
the overhead of gRPC tracing.

name                                                         old time/op    new time/op    delta
UnaryServerInterceptor/ok_no_metadata-8                        5.33µs ± 8%    4.94µs ± 5%   -7.17%  (p=0.002 n=9+10)
UnaryServerInterceptor/ok_with_metadata_no_parent-8            6.44µs ± 7%    5.79µs ± 3%  -10.19%  (p=0.000 n=9+9)
UnaryServerInterceptor/ok_with_metadata_with_parent-8          5.52µs ± 9%    5.05µs ± 3%   -8.47%  (p=0.000 n=10+9)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8    5.61µs ±10%    5.15µs ± 3%   -8.06%  (p=0.007 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                     13.3µs ±15%    11.7µs ± 0%  -12.16%  (p=0.007 n=10+6)

name                                                         old alloc/op   new alloc/op   delta
UnaryServerInterceptor/ok_no_metadata-8                        4.94kB ± 0%    4.89kB ± 0%   -1.00%  (p=0.000 n=9+9)
UnaryServerInterceptor/ok_with_metadata_no_parent-8            5.57kB ± 1%    5.52kB ± 0%   -0.88%  (p=0.000 n=10+9)
UnaryServerInterceptor/ok_with_metadata_with_parent-8          4.66kB ± 0%    4.62kB ± 0%   -0.97%  (p=0.000 n=10+9)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8    5.14kB ± 0%    5.09kB ± 1%   -0.94%  (p=0.000 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                     11.3kB ± 0%    11.2kB ± 0%   -0.33%  (p=0.004 n=10+10)

name                                                         old allocs/op  new allocs/op  delta
UnaryServerInterceptor/ok_no_metadata-8                          52.0 ± 0%      51.0 ± 0%   -1.92%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_with_metadata_no_parent-8              57.0 ± 0%      56.0 ± 0%   -1.75%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_with_metadata_with_parent-8            48.0 ± 0%      47.0 ± 0%   -2.08%  (p=0.000 n=10+10)
UnaryServerInterceptor/ok_no_metadata_with_analytics_rate-8      54.0 ± 0%      53.0 ± 0%   -1.85%  (p=0.000 n=10+10)
UnaryServerInterceptor/error_no_metadata-8                       69.0 ± 0%      68.0 ± 0%   -1.45%  (p=0.000 n=10+10)