Support for timestamp in text format

[erichard]

Hello,

The text exposition format is defined like this

metric_name [
  "{" label_name "=" `"` label_value `"` { "," label_name "=" `"` label_value `"` } [ "," ] "}"
] value [ timestamp ]

I haven't found a way with this library to specify the timestamp of my metrics. It could be great as it allow us to expose past metrics.