fix(instrumentation-web-exception): properly add bounded error listener

[BenoitZugmeyer]

Which problem is this PR solving?

The README.md advises to use the instrumentation like this:

registerInstrumentations({
  instrumentations: [
    new ExceptionInstrumentation({ ... }),
  ],
});

But this does not work because the function passed to addEventListener('error', ...) is the unbounded method, so accesses to this.getConfig() fails.

I know this sounds surprising, because the onError method was properly reaffected with the bounded method in the constructor. But actually, enable() is called by the base class, during super(), which is called before the onError is bound!

export class ExceptionInstrumentation extends InstrumentationBase<GlobalErrorsInstrumentationConfig> {
  constructor(config: GlobalErrorsInstrumentationConfig = {}) {
    super(PACKAGE_NAME, PACKAGE_VERSION, config); // calls this.enable()
    this.onError = this.onError.bind(this); // this is done too late!
  }

This wasn't caught in tests because in test, enable() is called two times, once when instantiating the instrumentation, and once explicitely when calling instr.enable(). This second call do register the correctly bounded method.

Short description of the changes

• To work around this, this PR make sure the onError method is proprely bound in enable().
• Adjust the tests to be closer to the documented setup instructions.

[wolfgangcodes]

Thanks @BenoitZugmeyer