Add timestamp option to render_sample

[j2ghz]

Fixes #17
This is a simple implementation where the parameter is Option<i64>. This leaves working with dates to the crate's users.
From prometheus' documentation:

The timestamp is an int64 (milliseconds since epoch, i.e. 1970-01-01 00:00:00 UTC, excluding leap seconds).

I think I could use https://docs.rs/chrono/0.4.11/chrono/struct.DateTime.html#method.timestamp_millis instead, so users can pass chrono's DateTime. The question is whether this is desirable, taking a hard dependency on chrono. Another option would be an enum with options of either a DateTime or an i64. (I will have to implement it for the exporter I'm writing anyway, so the question is just if it should be in this base crate, or not.) Let me know what you think @MindFlavor

[MindFlavor]

That's great! Thank you! I prefer your approach, forcing Chrono on users does not give us many advantages.