implement DiskUsage for the rest of the datastores

[Stebalien]

No description provided.

[hsanjuan]

hmm tests failing..

[Stebalien]

hmm tests failing..

we were using the wrong path for a package (path wasn't listed in the package's package.json)

I have no idea why this hasn't been failing for years. Maybe the test cache expired?

[codecov]

Codecov Report

Merging #86 into master will decrease coverage by 9.94%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
- Coverage   61.51%   51.57%   -9.95%     
==========================================
  Files           8       16       +8     
  Lines         408      952     +544     
==========================================
+ Hits          251      491     +240     
- Misses        141      429     +288     
- Partials       16       32      +16

Impacted Files	Coverage Δ
autobatch/autobatch.go	50% <0%> (-2.64%)	⬇️
retrystore/retrystore.go	78.94% <0%> (-21.06%)	⬇️
namespace/namespace.go	63.63% <0%> (ø)
keytransform/interface.go	33.33% <0%> (ø)
key.go	74.77% <0%> (ø)
basic_ds.go	0% <0%> (ø)
keytransform/keytransform.go	46.87% <0%> (ø)
batch.go	0% <0%> (ø)
datastore.go	0% <0%> (ø)
... and 1 more

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