feat: do not output timeseries whose resource_type does not match an allowlist

[nadiamoe]

First take at browser metric filtering. This PR adds an allowlist of values for the resource_type tag, which k6 browser uses to indicate what prompted a request.

The code roughly works as follows: After removing timeseries by metric name as it was previously done, the list of timeseries is walked again. If a timeseries contains the resource_type label, and the value for that label is not present in the allowlist, the timeseries is yeeted. The allowlist is case-insensitive.

The allowlist itself defaults to just document, which means HTTP requests initiated by loading a webpage. This allowlist can be configured using the SM_K6_BROWSER_RESOURCE_TYPES environment variable, as a comma-separated list of valid resource_types. The special value *, if present, allows all resource_types and thus skips this second walk entirely.

A couple of integration tests, with different levels of granularity are added, both to cover the removal functionality and the handling of the SM_K6_BROWSER_RESOURCE_TYPES variable.

The mechanism by which a user can specify this allowlist in the SM UI/API and then have that value traverse its way here is out of the scope of this PR. Whether we want to merge this (breaking) change before or after merging this is also out of scope: If we chose to merge this first, then we can make the change non-breaking to users by setting SM_K6_BROWSER_RESOURCE_TYPES=* in the runner.

[d0ugal]

This seems reasonable but its not clear to me how/where this will be used?

[nadiamoe]

@d0ugal I don't think that's decided yet. My personal preference is to enforce this default everywhere, meaning this change will propagate to every browser check in there automatically. As we're in private preview, we can get away with this, and furthermore this will fix many browser scripts that are seeing their metrics truncated as they produce too many timeseries. In the future, if people request this, we can allow end users to tweak this allowlist by adding a field for this in the check settings.

If for some reason we don't want to do the breaking part, there's also a nonbreaking route:

• Set SM_K6_BROWSER_RESOURCE_TYPES=* in the runner when we update xk6-sm to the version carrying this change (effectively negating the change)
• Develop the UI, API, Agent, and runner plumbing to allow users to specify this in the script settings, defaulting to "only document" for new checks.
• Use that property of the script to set SM_K6_BROWSER_RESOURCE_TYPES in the runner