Scripted metric aggregation support

[gmarz]

Adding support for the scripted metric aggeragtion

The tricky bit with mapping this aggregation is that the resulting value that the scripts return can be of different types and our current ValueMatric only supports the value being numeric (implemented as a double?).

As a solution, I came up with ScriptedValueMetric which holds the original object deserialized by Json.Net, and exposes ValueAs<T> which converts the value to the specified T. See these tests. Open to suggestions on a better way to handle this...

[Mpdreamz]

LGTM @gmarz, like the workaround you came up with 👍

Made some small tweaks before merging it in.

The object Value prop is now internal, do not think we should expose JObject directly even if its boxed as object. Renamed ValueAs<>() to Value<>().

[gmarz]

@Mpdreamz 👍 on making _Value internal. I actually had it that way originally, but was unsure.