Clarify pluralization guidelines for semantic conventions

[pmm-sumo]

What are you trying to achieve?

Most of the currently defined attributes use singular names as they typically address a single entity (e.g. os.type, db.connection.string, etc.) While it's possible to use a map or array as a value type, there are few examples found, such as process.command_line (even then it's not fully clear if arguments are always included as an array or if it's sometimes an array and sometimes a string)

There are cases (discussed in contrib#1167, where an attribute typically describes a single entity but sometimes might actually represent a collection of them.

I believe it might be helpful to clarify the rules for naming plural attributes. A similar initiative was taken for metric names recently.

Specifically, there might be two use-cases considered:

(I) Rules for attributes that are always expected to represent multiple entities

1. Should such attribute names always use plural name?
2. Should such value type be always an array?
3. What about maps?

(II) Rules for attributes that typically represent a single entity but sometimes represent a collection of them

1. Should two (singular and plural) attribute names be used (e.g. k8s.pod.container for a single instance and k8s.pod.containers for multiple instances)?
2. Or, should a single attribute name be rather used? In such case, should it be the singular or plural form, e.g. k8s.pod.container or k8s.pod.containers?
3. If a single attribute name is used, should it always use array type for the value or may it use string/number if only single value is present?

[arminru]

Related: #1053

[Oberon00]

While it's possible to use a map or array as a value type

Map is not possible, see #376.

[justinfoote]

Related: #1109

[andrewhsu]

from the spec sig mtg today, marking as P1 to get analysis of what is needed to change so a decision can be made if the spec needs a breaking change.

[pmm-sumo]

@bogdandrutu just wanted to confirm if I understood the suggested policy correctly; the way I understood conclusion from the SIG is following:

If the value can represent multiple entities, always use a plural name and store values in an array.

Did I get it right? In such case k8s.container.name would be renamed to k8s.pod.containers* it would be good to rename process.command_line to something else (process.command.arguments?)

* we seem to came to the conclusion it's probably not necessary in discussion under the other PR

[jmacd]

Focusing on the process.command_line points above. I do not believe there is any ambiguity in allowing this attribute to be written as a single string or as an array of strings. We are trying to define semantics, and I think we all know what is meant by a "process command line" whether it is presented as a string or as an array. In both cases (string or array), it's a singular value: one string, or one array. If you are writing code to handle this annotation, I would recommend that you have a switch statement and treat both well-understood cases. I recommend this because if the natural form of a command line is a string, then that's the encoding we should use. If the natural encoding is an array of strings, then that's the encoding we should use. They mean the same thing either way.

[jmacd]

In contrib#1167: it's not clear whether the k8s.container.name being discussed is the same concept hypothetically being described as a plural. I've asked for examples when k8s.container.name might have multiple values, but like the command_line example above, I think of attributes as single-valued whether it's one container name or one list of container names.

In #1053: here we have an inconsistency about "size" vs. "length". We have discussed this inconsistency as relating to their domains: HTTP uses "length", while messaging systems use "size". I would propose we leave these inconsistencies. These are both singular nouns, in any case.