Opentelemetry-specification: Define metric semantic conventions for FaaS

[jgals]

What are you trying to achieve?
Defined semantic conventions for function as a service (FaaS) metrics, often referred to as "serverless" metrics.

What did you expect to see?
FaaS metric semantic convention documentation in markdown generated from YAML definitions as per #547.

Additional context.
These should be roughly analogous to the trace semantic conventions for FaaS spans defined here. For reference, metric semantic conventions for HTTP operations are already defined here.

[jgals]

Hey @jmacd, can we add @kolanos to the project so he can be assigned to this issue? I believe he's working on a PR.

[Oberon00]

@kolanos just needs to comment on this issue, then he can be assigned.

[kolanos]

@Oberon00 Commented. :)

[skonto]

Hi @kolanos are you still involved in this issue? I am interested in defining the related faas metrics as I am working on defining these for Knative.

[skonto]

Hi @jmacd is there a way to push forward this? I think @kolanos has something as wip but if there are no cycles I can pick it up.

[kolanos]

@skonto If you want to pick up where I left off, I'm +1. Happy to provide feedback as well.