Avoid redundant metadata reads in ZarrArrayWrapper

[olimcc]

Modify ZarrArrayWrapper to avoid re-reading metadata each time data is read from the array.

• Closes Potential performance optimization for Zarr backend #8290
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

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[jhamman]

Small suggestion but this looks great. No doubt this is going to really improve performance when reading data with zarr.

[headtr1ck]

I think this looks good, thanks!

Maybe some get_array() calls can be replaced by _array, but that's really minor.

You should add an entry in whats-new!

And as a bonus you could add a benchmark, but that's really optional (seems like there are only netcdf Io benchmarks right now).

[olimcc]

Thanks @headtr1ck!

You should add an entry in whats-new!

Great, this has been done.

And as a bonus you could add a benchmark, but that's really optional (seems like there are only netcdf Io benchmarks right now).

I didn't do this. I'll try to add a note to #8290 of a before/after scenario, to illustrate the impact of the reduced requests.

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