Expand marker schema to allow dynamic label

Currently the schema for marker tooltip rows only allows `label: value` lines where `value` may be static or dynamic (i.e., taken from the marker data payload), but the `label` can only be a fixed string.

It would be useful in some cases to allow that label to also be dynamic. This was suggested [here](https://phabricator.services.mozilla.com/D135031#inline-742304), where some code wants to display key+value pairs coming from an API, where both keys and values are only known at runtime.

This would require some work on the backend as well. When someone can work on this here, please [file a corresponding bug there](https://bugzilla.mozilla.org/enter_bug.cgi?product=Core&component=Gecko+Profiler).

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-498)