Add OT-to-AO support for Method, Status, URL aggregate metrics feature

[cce]

When customers use our builtin http.Handler instrumentation, or call methods on ao.Trace() they get metrics aggregation for throughput, latency, and error counts aggregated by transaction name, HTTP method, and HTTP status.

This PR allows OpenTracing users to access those aggregation features by calling OT Span.SetTag() on http.url, http.method, and http.status_code. (Support for setting transaction name for use in these metrics aggregations was part of #147, but did not cover HTTP method and status.)

[codecov-commenter]

Codecov Report

Merging #153 into master will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
+ Coverage   82.05%   82.19%   +0.14%     
==========================================
  Files          37       37              
  Lines        4603     4629      +26     
==========================================
+ Hits         3777     3805      +28     
+ Misses        703      700       -3     
- Partials      123      124       +1     

Impacted Files	Coverage Δ
v1/ao/opentracing/tracer.go	100.00% <100.00%> (ø)
v1/ao/internal/reporter/reporter_grpc.go	70.18% <0.00%> (-0.31%)	⬇️
v1/ao/trace.go	86.25% <0.00%> (+2.50%)	⬆️

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[jiwen624]

it looks good from what I can tell 👍