[batchprocessor] Batch processor reimplemented with Exporterhelper

[jmacd]

Description

Part of #8122
This replaces the batchprocessor's legacy implementation with one based on exporterhelper.

Two featuregates are proposed:

1. useExporterHelper: use the exporterhelper-based implementation
2. propagateErrors: propagate errors from the batcher through to clients

Note the new implementation has been tested except for the metadata_keys-related support (#10825). The only problem here is that the component outputs exporter-like metrics, not legacy-like metrics. The failing tests are because of this detail. We could register metric views to rename exporterhelper metrics to processor metrics maybe, but that will be a little ugly and I would prefer to have an explicit metric-naming override.

Part of #13478: we need the batch-size metric which is used in existing tests by the batch processor.

Link to tracking issue

#8122 see also #10825

Testing

Yes [WIP]

Documentation

TODO