ByteStreamBuildEventArtifactUploader: skip reading metadata for files that won't be uploaded

[christianscott]

Note

draft because I'm not yet sure about the implementation

54% of samples in a profile I took of our build included readPathMetadata()

as far as I can tell, path metadata is only needed for files that are going to be uploaded to the CAS. for builds that use minimal remote uploads, this results in a lot of extra unneeded stats.

this makes a fully cached remote build (cold local) of a large subset of our monorepo 3x faster. I am not totally sure why given that (I think) this function is only used by the bes-uploader thread, but the working theory is that this allows BEP events to be processed more quickly, which keeps the eventQueue small and reducing memory usage

tested with Bazel 6.4.0. have not tested this change because our monorepo is not compatible with Bazel 7.x yet.

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[KipHamiltons]

What do we need for this one to get over the line @christianscott ?