[OTLP Registry] Document batch size configuration

[joaopgrassi]

Please describe the feature request.
The current docs only mention the step config which is the equivalent to the export interval in OTel SDKs. But I saw that there's also a "batch size" which would also be nice to have in the docs.

Rationale
Usually these two properties go together so I think it's a good idea to demonstrate to users that those options are available.

Additional context
It is common for metric back-ends to have limits on the size or/and the amount of metrics in a single export request. With both the interval and batch size, users can tweak things to make sure their metric requests stay under the limits.

[shakuzen]

Thanks for bringing it to our attention. A pull request would be welcome. As a side note, we have been thinking about ways to keep the docs about configuration options in sync with the code but we haven't implemented anything so far, so these things happen from time to time.