Add microbenchmarks for span creation and annotation (#474)
Add microbenchmarks for span creation and annotation (#474)

BenchmarkStartEndSpan-8                  10000000   185 ns/op   128 B/op    2 allocs/op
BenchmarkSpanWithAnnotations_3-8   5000000	    338 ns/op   224 B/op   5 allocs/op
BenchmarkSpanWithAnnotations_6-8   3000000	    475 ns/op   320 B/op   8 allocs/op
census-instrumentation/opencensus-go,census-instrumentation/opencensus-go,census-instrumentation/opencensus-go