Rename `source` in `MetricSample` to `connection`

[tiyash-basu-frequenz]

What's needed?

The use-case for this field is to name the electrical connection for a metric within a component, e.g., a PV inverter's DC string name. In this regard, connection sounds like a better choice than source.

Proposed solution

No response

Use cases

No response

Alternatives and workarounds

No response

Additional context

See #258 to see more proposed changes to metric sources.

[tiyash-basu-frequenz]

If we decide on doing #339 , then we need to rethink this, since source is more gebneric, and would apply to both sensors and electrical componets.

[thomas-nicolai-frequenz]

OK, so we decided to go with connection. Can this issue be closed @tiyash-basu-frequenz ?

[tiyash-basu-frequenz]

My original thought about keeping source was that we keep it generic enough, so that it stays flexible for applying to wider use-cases, like sensors.
But if a sensor has, say, multiple temperature sensors, then those would still be "connected" to the central sensor hardware. So, after all, connection applies to sensors too.

@thomas-nicolai-frequenz this issue will be automatically closed when #347 gets merged.