Add URL quantization callback support to Sinatra instrumentation

[fbogsany]

Our Rack instrumentation allows configuration of a URL quantization function https://github.com/open-telemetry/opentelemetry-ruby/blob/1c37b00dc82d490dace1f92739977aa4674d7039/instrumentation/rack/lib/opentelemetry/instrumentation/rack/middlewares/tracer_middleware.rb#L122-L137

We should add the same functionality to the Sinatra instrumentation. @genebean reported this requirement in Gitter:

I am using the auto instrumentation of the Sinatra but need to account for url paramters so that /api/v2/status/queue/info/1600289926679 is instead traced as /api/v2/status/queue/info/{job_id} - any tips on how to do this?

[genebean]

Thanks! Please make sure that instrumentation/rack/README.md and instrumentation/sinatra/README.md are updated to include how to put that to use... I don't see anything about it at this time.

[mwear]

We can do this, but I think we should also use env['sinatra.route'] as the span name (by default). It is already a quantized low-cardinality string. For example, for this route:

get '/accounts/:id' do
  ...
end

sinatra.route for a request with an actual id will always be "GET /accounts/:id" .

[genebean]

@mwear would that be as simple as editing https://github.com/open-telemetry/opentelemetry-ruby/blob/master/instrumentation/sinatra/lib/opentelemetry/instrumentation/sinatra/middlewares/tracer_middleware.rb and replacing env['PATH_INFO'] with env['sinatra.route']? If so, I will put up a PR

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