UserAgent is the user agent to be added to the outgoing requests from the exporters.
MonotonicEndTime returns the end time at present but offset from start, monotonically.
The monotonic clock is used in subtractions hence the duration since start added back to start gives end as a monotonic time. See https://golang.org/pkg/time/#hdr-Monotonic_Clocks
Sanitize returns a string that is trunacated to 100 characters if it's too long, and replaces non-alphanumeric characters to underscores.