fix graphql integration not working when using schema stitching

[rochdev]

This PR fixes the graphql integration not working when using schema stitching. This is a feature of Apollo Server that allows merging multiple schemas together.

This was not working before because the instrumentation was storing metadata on the schema and the contextValue with the assumption that there is only one of each for every operation. Now all metadata is properly attached to the operation itself and is thus independent from the number of schemas or changes to contextValue.

Fixes #235 #307

[tylerbenson]

You should consider writing a test library to make your trace testing more declarative like Java.

[rochdev]

@tylerbenson Thanks for the suggestion! Do you have any ideas for the syntax of the test helper?

[tylerbenson]

@rochdev not really in particular. Perhaps some sort of JSON like document structure? That's kind of what ours looks like.

[rochdev]

@tylerbenson I guess it could already be done with deep.equal.

For example:

expect(span).to.deep.equal({
  service: 'test',
  resource: 'GET /user/:id',
  meta: {
    'http.url': 'http://localhost/user/123'
  }
})

I actually don't know why I haven't been using that for all tests (some do).