move otel-collector to its own repo

[madwort]

• this has grown a bit & it may now be more convenient to manage in its own repo
• we recently had an issue where broken tests in an unrelated CI test in the sysadmin repo blocked us from merging an urgent update to otel-collector

[madwort]

had exactly this problem this week with #178 (fixed by #181 )

[madwort]

alternatively, should we improve CI so that it doesn't run unnecessary tests?

[madwort]

One thing is that I think some other parts of the organisation assume a one-to-one mapping from repository to "pieces of software", and so didn't realise that the otel-collector even existed!