Remove channel registry

[simonvoelcker]

This removes the ChannelRegistry class.

Background

This is part of an initiative to close unused channels to save resources. The ChannelRegistry was a de-facto singleton class for storing channels by name. Request objects such as ComponentMetricRequest or ReportRequest have a get_channel_name() method, and the returned channel name acted as a contract between sender and receiver - With the name, they could obtain the same channel from the ChannelRegistry. By getting rid of the registry, we are moving towards a target state where only references to senders and receivers, but no channels are being held.

Implementation

ComponentMetricRequest

ComponentMetricRequest objects represent a request to receive metrics from a component - Sometimes also referred to as telemetry stream. To serve such a request without relying on the ChannelRegistry, the request got a new attribute: telem_stream_sender: Sender[Receiver[Sample[Quantity]]]. This is the sender of a one-shot channel that is used to send the receiver of the telemetry stream back to the original requester - Similar to a callback function. The receiver of a component metric request should therefore create (and own) the channel that sends the metric data, and use the one-shot channel to send a receiver of that channel "back".

In a few places, component metric requests were created and sent from synchronous functions which returned the receiver. To make this work with the round-trip outlined above, an additional channel was added that merely forwards the metric data via a Pipe. This allows us to immediately return a sender while the channel that serves the metric data is being constructed asynchronously.

These changes can be found in the first commit.

ReportRequest

ReportRequest objects are a different kind of request that was handled similarly.

These changes can be found in the second commit.

Channel ownership

The receivers of request objects create the channels and own them. For instance, in case of ComponentMetricRequests, MicrogridApiSource now maintains a dictionary (channel_lookup: dict[str, Broadcast]) as a replacement for the channel registry. It is necessary to have this lookup dictionary to avoid creating duplicate channels for similar metric requests. In this case, a new receiver is created from an existing channel found in the lookup dictionary.

Forwarding channels are owned by their creator (e.g. GridFrequency). These (and the Pipes that connect them to the other channels) are currently never closed. Closing these when all receivers or senders are closed is a TODO for a later iteration.

[simonvoelcker]

I won't merge before this is merged 👀