Consider allowing to add/remove receivers from Select dynamically

[leandro-lucarella-frequenz]

We have some use cases where we need to add/remove receivers to a Select inside the .ready() loop. For now we need to create a second loop where Select is created, then break the inside (.ready()) loop and re-generate the Select object, which is inconvenient from the POV of the code, and also makes the code harder to read, as the flow becomes more complicated.

Another big issue with this approach is if it's not done very carefully, we can lose messages from receivers if the break is done before all available messages from all receivers in the current loop are not handled. The break should always be the last in the loop.

This also applies to Merge and MergeNamed.

Example:

        while True:
            receivers = await gen_component_receivers(
                self.component_infos, self.microgrid_client
            )


            # Create a task that will periodically send formula results.
            resend_formulas_task = asyncio.create_task(self.resend_formulas())
            select = Select(
                component_data_receiver=Merge(*receivers.values()),
                config_update_receiver=self._config_update_receiver,
            )
            while await select.ready():
                if msg := select.component_data_receiver:
                    # Update symbols and recalculate formulas.
                    updated_symbols = self.formula_calculator.update_symbol_values(
                        msg.inner
                    )
                    self.formula_calculator.compute(
                        updated_symbols, only_formula_names=set(self._outputs.keys())
                    )
                elif msg := select.config_update_receiver:
                    try:
                        await self._reinitialize(msg.inner, resend_formulas_task)
                    except Exception as err:  # pylint: disable=broad-except
                        logger.error(
                            "Reinitialization failed, because an error occurred "
                            "while applying the new configuration: %s",
                            err,
                        )
                    else:
                        # Break from the inner loop to regenerate component receivers
                        break

https://github.com/frequenz-floss/frequenz-sdk-python/blob/4ce090ce32f539014101f145e8f97fc67990fa9e/src/frequenz/sdk/data_ingestion/microgrid_data.py#L229-L260

Other use case came from here:

• Add a resampling actor frequenz-sdk-python#32 (comment)

[leandro-lucarella-frequenz]

This will probably be closed in favor of #29, as we don't want to add new ways to misuse Select, but I will leave it open until the docs are ready just in case.

[leandro-lucarella-frequenz]

We will not pursue this feature and improve documentation (#29) to make it clear that Select has a very specific and narrow use case and shouldn't be used if this kind of flexibility is needed.

[llucax]

The current proposal is to implement this externally, by adding support to add/remove channels to a merge as defined in:

• Improve composability of receivers #116