Bug: Tracer ignores configured sampling rate

[atreidesend]

It looks like the tracer constructed in opentracing/tracer.js is ignoring the configured sample rate and using a hardcoded value of 1

this._sampler = new Sampler(1)

Unless I'm missing something this samples everything, regardless of config. Happy to put together a quick PR to patch -> it could probably use something as simple config.sampleRate || 1

[rochdev]

Thank you for reporting this oversight! Since we are planning a release in a few hours, I will make the PR to make sure I can get it to go in that release.