Add a prefix to the browser module metrics

[ankur22]

The browser module emits metrics. One such metric(s) are http ones. k6 itself emits http metrics when users use the http module.

The implementation of the http client in the http module vs what the browser modules users differs which can result in different measurements. This will affect aggregations across the http metrics.

Users are likely to want to measure and analyse the http metrics from the http module and browser module separately rather than aggregated together too.

There maybe other such metrics where there's a clash between what the browser module emits and what existing k6 modules emit.

We feel that it's best to rename the existing browser module metrics so that we avoid this clash and allow a more flexible way of measuring and analysing metrics from different modules.

So, for this issue, we need to prefix the existing emitted metrics from the browser module with browser_ where we're not already doing so.

[ankur22]

At the moment it seems that k6 seems to continue to report on builtin k6 metrics even if the browser module isn't using them. I've created a new browser_data_sent metric in the browser module (a clone of the data_sent metric in the k6 module), and in the test summary we see this now:

     browser_data_sent.........................: 2.6 kB  1.6 kB/s
     ...
     data_sent.................................: 0 B     0 B/s
     ...

We need to find a way to stop the metrics that aren't used from being displayed.