Update covidcast.metadata() return values in docstring

[chinandrew]

Currently the docstring is missing a few columns which get returned, and is also out of order

documented:
data_source signal min_time max_time geo_type time_type num_locations min_value max_value mean_value stdev_value

returned (with example row:

        data_source                 signal time_type geo_type   min_time   max_time  num_locations  min_value  max_value  mean_value  stdev_value  last_update  max_issue  min_lag  max_lag
0     doctor-visits       smoothed_adj_cli       day   county 2020-02-01 2020-10-01           2514    0.00000  87.670832    2.674555     3.358741   1601853923   20201004        2      129

capnrefsmmat I'm not actually sure what those last 4 undocumented columns mean, is there a reference somewhere? Is the last updated in unix time, and if so, is that intentional? (1601853923 = Oct 4)

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[chinandrew]

I think I found the answers here

Should we convert the unix time to a date[time] object?

[capnrefsmmat]

Yeah, I think that'd be helpful. It's unfortunate that a Unix timestamp crept into the metadata, so let's convert it to a datetime.

[chinandrew]

👍 Done in #118