Manage different types appropriately in filters

[colmsnowplow]

As it stands, we use fmt.Sprint() to coerce non-string values to string for purposes of evaluating via regex. This is obviously not brilliant, but we get away with it because a) typically people will mostly need string and int, b) we know what other types will be cast to, and c) scripting transformations can be used to cover other cases if need be.

We should design and instrument something better for other types - off the top of my head I think we could either go with a) a separate filter for different types, or b) handling different types explicitly in the existing filter. Not sure which is better.