Document the purpose and usage of client metadata

[1pkg]

Context

elasticapmconnector and other collector components are making use of client metadata. But there is no explicit definition of it inside the documentation, which can lead to a confusion and misinterpretation.

Goal

Better document and explain the purpose and how client metadata is used in EDOT.

[theletterf]

@alexandra5000 Assigning this one to you.

[colleenmcginnis]

@theletterf can we leave this unassigned until we do our next round of sprint planning?

[theletterf]

Sure.