We're going to bundle the web vital js library from the internet
instead of downloading them on each test run. This will ensure that
users who run tests in a locked down environment can still work with
web vitals, and it will avoid any CORS related issues. The downside
is that we will need to manually update this library, and users who
do not update to latest k6 binary will be working with an outdated
version of the library.

This will reduce the complexity of the method.

This will evaluate the script on the current page. BrowserContext
relies on page.evaluateOnNewDocument when a new init script is added
so that they are applied to all existing pages.

For now, the preference is not to work with the worldName param in the
AddScriptToEvaluateOnNewDocument CDP command. For now we don't want
the script to be isolated to a particular named world.

The script identifier that is returned from a successful CDP command
to add the init script, is ignored. When we need it (e.g. for removing
init scripts) we should use it then.

Co-authored-by: Daniel Jiménez <daniel.jimenez@grafana.com>

When a new page is created, it needs to add the init scripts that the
browser context has had applied to it.

Instead of retrieving the list of scripts to apply in the page, we're
sending the current page to the browser context, and it will loop over
the scripts it has in evaluateOnNewDocumentSources.

Bindings are a CDP runtime feature, where we can bind the current
frame with a method on the window object in the browser. When the
method is called with a payload, the listener in frame_session.go will
currently read from the event and logs the output.

By refactoring this one method, we need to follow the chain up so that
other methods that work with NewBrowserContext also return an error.
We need NewBrowserContext to return an error so that we can validate
the web vital init script ,which is currently being added to
evaluateOnNewDocumentSources without any validation (change to come in
the next commit).

Now that we can return errors from NewBrowserContext, we can also
validate the web vital script. We might as well reuse the
AddInitScript to help us validate and add the web vital script to
evaluateOnNewDocumentSources and ready for use by the new pages.

The Web Vital init script glues together the existing change. We first
import the web vital lib and web vital init scripts into the website
when a new page is created. The init script uses the web vital lib
script which has been predownloaded, and the binding setup earlier to
send the web vital metric payload to xk6-browser. At the moment the
metric payload is logged (debug only).

This adds the six web vitals, as well as the three ratings for each.
When a web vital metric is send from the JS code to k6-browser via
the binding, it will need to extract the metric name and the rating.

The rating can be good, needs-improvement or poor.

This unmarshalls the web vital metric from the JS script; does some
error checking (ensuring that the metric is recognized and is
registered), and finally emits the metric.

We're now getting web vital metrics which are calculated by the JS
web vital script. It also contains first contentful paint FCP metric.
The metric coming from the JS lib also comes back with a rating. We
can't correlate that with the current CDP FCP. To keep web vital
metrics source consistent, we will remove the existing CDP FCP and opt
to work with the one coming from the JS lib.

Co-authored-by: İnanç Gümüş <inanc.gumus@grafana.com>
Co-authored-by: Daniel Jiménez <daniel.jimenez@grafana.com>

This will hopefully help highlight that it is an internal k6-browser
function that was defined by us, and also not be used by anyone else.

Resolves: #836 (comment)

Resolves: #836 (comment)

It was using POC code to import the module directly from the internet
instead of using the library that was downloaded and embedded in the
k6-browser binary and initialised when the browser context was
created.

Resolves: #836 (comment)

This test ensures that the embedded web vital scripts have been added
to the browser contexts internal evaluateOnNewDocumentSources slice.
It does not check whether this has then been applied to a page.

This allows us to read off the metric sample channel when it gets
populated with metrics during a test run. This will be useful to
assert that certain metrics have been emitted during a test.

This tests ensures that web vitals are measured and emitted to the
metric samples channel. We read off the channel and assert that
expected web vitals metrics have been emitted during the test run.