Disable MetadataComponent for local development

[janheinrichmerker]

Is your feature request related to a problem? Please describe.
The MetadataComponent, as I understand it, speeds up querying metadata in production by having a pre-computed metadata index.
But when developing doc classes for new datasets, this component often intercepts calls to docs_count etc, making it difficult to trace down issues in the development code (because the MetadataComponent sort of monkey-patches the corresponding functions).

Describe the solution you'd like
I'd be happy to have some opt-out environment variable to effectively switch off the monkey patching in this line:

ir_datasets/ir_datasets/util/registry.py

Lines 41 to 42 in 546baf8

	if not hasattr(obj, 'metadata'):
	obj = Dataset(MetadataComponent(name, obj), obj) # add metadata from default provider

Describe alternatives you've considered
Alternatively, the metadata component should at least have clear error messages that tell developers that some metadata is missing and how to add those.

Additional context
For developing integrations for large datasets, e.g., #213, having to generate metadata while developing the parsers is also not that great as one would often test the parsers on a smaller sample before computing metadata on the whole database.