Add support for exemplars

[jkylling]

It would be useful to support observing exemplars on the default metrics, for instance to have trace IDs as exemplars.

We could add another function ExemplarFunc func(c *gin.Context) string to the Prometheus struct. If the function is non-nil, and the string it returns is non-nil, then the exemplar returned is used when incrementing and observing the request count, duration and sizes. This would enable having trace IDs as exemplars, which can make it simpler to connect metrics to traces.

There is an implementation using exemplars at https://github.com/wei840222/gin-prometheus which is a bit less configurable.