canonical_labels(
labels) ->labels_str,err
Canonicalize a labels table into the wire labels string
Validates label keys against [a-z0-9_]{1,32}, escapes values (max
128 bytes before escaping), sorts pairs by key and joins them as
key=value,.... A nil or empty table yields the empty string. Series
identity relies on this canonical form: the same table always encodes
to the same string.