Add tagged metrics support to whisper converter

[ywwg]

(imported issue)

Right now the whisper converter only supports untagged metrics. We should add support for tagged metrics.

From the docs, the tags are stored in a separate database from the whisper files. So it looks like:

- [ ] We will need an example tagdb, preferably in sqlite, but it looks like the tagdb can be any sql db and therefore the converter would need to be able to connect to an SQL server.  (Or can we insist that they provide an sqlite file? Or can we ask them to dump a CSV? That CSV might be GIGANTIC so probably not)
- [ ] in Pass1, when we hit a metric we need to query the tagdb -- I am not able yet to find the schema / API... maybe this:https://github.com/graphite-project/graphite-web/blob/master/webapp/graphite/tags/models.py
- [ ] once we have the tags for a metric it's easy to call LabelsFromTaggedName

[ywwg]

I think based on the schema we'd do a big JOIN, something like
SELECT Tag.tag FROM Tag JOIN SeriesTag ON SeriesTag.tag == Tag.id AND SeriesTag.Series == Series.id WHERE Series.path=="METRICNAME"

and then

SELECT Tag.tag, TagValue.value FROM TagValue JOIN SeriesTag ON SeriesTag.tag == Tag.id AND SeriesTag.TagValue == TagValue.id WHERE Tag.id IN "THE RESULTS FROM ABOVE"

[ywwg]

ah, it'll be simpler than that, we don't need the tag values

[ywwg]

hm, the db may just be for querying. the actual storage appears to be an alternative scheme: https://github.com/graphite-project/graphite-web/blob/master/webapp/graphite/tags/utils.py#L125-L134