The TagValues API will perform a linear scan if there is no predicate;
otherwise, it will use the index to find a list of candidate series
keys.
TagValues expects the predicate to be transformed such that
`_measurement` and `_field` are remapped to `\x00` and `\xff`
respectively.
There is one TODO marked to analyze the predicate for a
`\x00 = '<measurement>'` pattern. If found, the predicate can be
eliminated and fall back to a linear prefix scan by combining the org,
bucket and measurement.
Stuart Carnie