fix: inject prometheus multiproc_dir into env var when worker starts

[frostming]

Signed-off-by: Frost Ming me@frostming.com

What does this PR address?

Fixes #(issue)

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[aarnphm]

my only concern with this is that users might override this envvar, and it would break bentoml.

I think the reason we have bentoml.metrics is for separation of concern. If users want to use prometheus_client then it is on the users to do their metrics, whereas with bentoml.metrics it falls under bentoml