Document how to support opentracing in functions

[LucasRoesler]

Proposed change

We should discuss and hint at how to support opentracing or tracing in general for functions. This is not about tracing the internals of OpenFaaS but supporting developers to add tracing to their functions.

Context

This has been asked about several times in Slack and also here openfaas/faas-netes#272

[alexellis]

Ping @LucasRoesler

[blaisep]

Hi @alexellis , @LucasRoesler , I'm just starting to look into the matter of observability in OpenFAAS, and I'm noticing the need for a pattern that {converges, includes, associates} metrics, logs and tracing. I think this will be particularly helpful in the context of OpenFAAS cloud, which introduces Tenancy and Accounting as additional dimensions.

[alexellis]

I'd be interested in seeing you write up a short tutorial using Tempo. What do you think @LucasRoesler ?