Add metadata to custom spans

[NoiseByNorthwest]

Since #110 it is possible to define the profiled spans at runtime through 2 dedicated functions.

These custom spans are however hard to identify on UI side (i.e. report list) since many of them can have mostly the same process level metadata, especially the full command line.

So we may rethink the profile report metadata structure and make possible, when starting a new profiled span, to inject some metadata to easily identify this span on report UI side.