Option to omit timestamp

[anegrin]

In prometheus exposition format timestamps are not mandatory; here's my current setup:

• ESP32 is emitting external temp and hum to MQTT and then going into deep sleep for 15 minutes
• Omitting the timestamp let the scraping "to emit" a value on each scrape

I opted for 0 time value instead of playing with pointers and nil

[hikhvar]

I see the use case, and I like the implementation. Could you please add that option to the example configuration in the readme with a proper comment?