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Added `collapse` keyword argument to `series()` for collapsing an entire
timeseries into a single row. Transforms are performed after the collapse,
allowing easy calculation of aggregates across an entire timeseries.
Fix inconsistency in gauge type when collapsing or condensing gauge data.
Value for an interval is the last recorded non-null gauge value now.
Added support in `get()` and `series()` for joining multiple timeseries
into a single result by calling with a list of timeseries names rather
than a single string.