Add cloud metadata to agent development docs

[basepi]

Ref #256

Agent	Milestone	Link to agent implementation issue
.NET	7.10	elastic/apm-agent-dotnet#918
Go	7.10	elastic/apm-agent-go#786
Java	7.10	elastic/apm-agent-java#1264
Node.js	7.10	elastic/apm-agent-nodejs#1815
PHP	7.10	elastic/apm-agent-php#76
Python	7.9	elastic/apm-agent-python#822
Ruby	7.10	elastic/apm-agent-ruby#841
RUM	n/a	n/a

[felixbarny]

Once #289 is resolved, I'd also add the option name, the values, the default and the description to this PR.

I'd also add some more detail around how the metadata can be obtained or alternatively link to python as the reference implementation. Also add that Delay intake API requests until with have computed the metadata (queuing events in the mean time) is the recommended way to init the meta data.