Add category attribute to spans

[justinfoote]

What are you trying to achieve?

We have an implied span category -- that is, spans can be either database, faas, messaging, etc...
But at the moment, the only way to determine what category a span belongs to is by inspecting its attributes.

I'd like to see a span.category added to the semantic conventions for spans.
I would expect its value to be an enum of:

database
faas
http
messaging
rpc

I suggest that this field is optional in general, but should be required for any span that falls into one of these categories. It should be omitted for general spans with a Span.Kind of INTERNAL.

Additional context.

This came up in the discussion on PR 1142 here.

[justinfoote]

@Oberon00

[Oberon00]

Older related discussions:

• Span name: Both low-cardinality (grouping key) and human-readable (display name) #557 "Span name: Both low-cardinality (grouping key) and human-readable (display name)"; resolution was that it is primarily a grouping key
• Clarify usage of "component" attribute in semantic conventions #336 "Clarify usage of "component" attribute in semantic conventions" and Do we still need component in semantic conventions? #271 "Do we still need component in semantic conventions?"; component was similar to span.category, but was removed in Remove component attribute from semantic conventions #447. AFAIK the reason (not documented, only in the SIG meeting 😕) was that (1) a span may have more than one category/component and (2) that component(s) can be determined by enumerating the attributes and looking a the part before the first . basically.