Optimize open-census usage

[howardjohn]

See census-instrumentation/opencensus-go#1265

Benchmark:

name       old time/op    new time/op    delta
Counter-6     713ns ± 8%     680ns ± 3%   -4.63%  (p=0.006 n=10+9)

name       old alloc/op   new alloc/op   delta
Counter-6      205B ± 0%      173B ± 0%  -15.63%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
Counter-6      6.00 ± 0%      5.00 ± 0%  -16.67%  (p=0.000 n=10+10)

This is not that much faster, but once my OC PRs merge we can take advantage of more optimizations this way, bringing us down to:

name       old time/op    new time/op    delta
Counter-6     713ns ± 8%     618ns ± 6%  -13.35%  (p=0.000 n=10+9)

name       old alloc/op   new alloc/op   delta
Counter-6      205B ± 0%       68B ± 1%  -66.59%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
Counter-6      6.00 ± 0%      3.00 ± 0%  -50.00%  (p=0.000 n=10+10)
``

[douglas-reid]

Looks OK to me. Thanks for tackling!

[douglas-reid]

This technically violates the explicit documentation for context.Context:

Do not store Contexts inside a struct type; instead, pass a Context explicitly to each function that needs it.

But I'm not sure that matters for this use case. This is a vestige of the OC library construction, and is not holding request-scoped information or any sort of deadlines, etc.