Allow `page.on('metric')` to create new tags

[inancgumus]

What

Use page.on('metric') for creating new name=value tags.

One way could be:

page.on('metric', m => {
  if (activeTiming) {
    m.tag({
        tags: [{name: 'foo', value: 'bar'}],
        matches: [{...url patterns to match...}],
    });
  }
});

Why

We want to add new tags to group URLs.

Due to #2728, group doesn't work in an async way, and we're looking for a low-hanging fruit to provide support for URL grouping. The current page.on('metric') API only allows replacing the values of url and name tags, but not their names.

How

• We need to discuss whether this is doable.
• Adding a new field, such as tags, can tell us whether to create a new tag. If tags exist, then, instead of replacing the values, we can add these new tags for the URLs that match the patterns specified in matches.