Profiling: Allow children profiling in prometheus without parents being profiled

What

There are cases (like in block_store.ml) where we have profiling such that:

f_1 [@profiler.span_s ["f"; "f_1"]]

f_2 [@profiler.span_s ["f"; "f_2"]]

But no @profiler.span_s ["f"].

(all of them having metadata = "prometheus")

In this case, the metrics observed are not updated, so after offline discussions with @julien.sagot.nl we decided to remove this constraint on the children profiling for prometheus. (this can definitely be improved, but given the tight schedule and desire to show results in Grafana, I really think we should stick with this for now)

Why

How

Manually testing the MR

In the metrics endpoint, check that with this MR, the output for profiling_block_validator_time section is updated, and without it, it's not.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR