Add "warnings" from prometheus upstream

[jacksontj]

Split from #141 (replacement of #168)
TLDR; upstream prometheus added a "storage.Warnings" which it uses for non-primary datasource reads. This pr is for adding support for such a thing

TODO:

• support "merging" warnings
• add "make_warnings" option, similar to "ignore_error" except instead of ignoring we make them warnings
• add "make_errors" option, similar to "make_warnings" except we take warnings and make them errors
• add warnings to NodeReplacer

References/links:

• Initial add of warnings to client (Add storage.Warnings to client prometheus/client_golang#562 )
• rework of warnings in api client (Pass warnings through on non-error responses prometheus/client_golang#599)
• add warnings to series (Add warnings to series prometheus/client_golang#603)
• labelsName/labelValues warnings (LabelValues and LabelNames missing storage.Warnings prometheus/prometheus#5661) (Add Warnings to LabelValues and LabelNames prometheus/client_golang#609)
• better configuration for warnings (Option to make failures of non-primary reads not warnings prometheus/prometheus#5662)