Disable MetadataComponent for local development

**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:
https://github.com/allenai/ir_datasets/blob/546baf8c40394c77de177060e3c58fef2ab6fdd7/ir_datasets/util/registry.py#L41-L42

**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.