[http-plugin] Missing traceparent header if WHATWG URL is used

[Flarna]

What version of OpenTelemetry are you using?

0.2.0

What version of Node are you using?

10.17.0

What did you do?

Use a WHATWG URL (https://nodejs.org/dist/latest-v10.x/docs/api/url.html#url_class_url) in http.get() instead an options object or string.

e.g. in https://github.com/open-telemetry/opentelemetry-js/blob/v0.2.0/examples/http/client.js#L22 replace
http.get({ host: 'localhost', port: 8080, path: '/helloworld' }
by
http.get(new url.URL('http://localhost:8080/helloworld').

Also when using WHATWG URL + options like this:
http.get(new url.URL('http://localhost:8080/helloworld'), { headers: { 'x-foo': 'bar' } }
traceparent header is missing.

What did you expect to see?

traceparent header should be present

What did you see instead?

traceparent header is missing

Additional context

a missing traceparent header is causing disconnected spans.

The current implementation handles WHATWG URL objects like an options object and attaches a headers property to it. But node core is ignoring a headers property on WHATWG URL objects (see https://github.com/nodejs/node/blob/v12.13.1/lib/_http_client.js#L96 and https://github.com/nodejs/node/blob/v12.13.1/lib/internal/url.js#L1257.

If wanted I can try to create a PR to fix this.

[mayurkale22]

Thanks for reporting this! Feel free to attempt to write solution and open a PR. :)