Expose a logger in pipeline/How do you take action on ErrDropped

[RugTumpington]

nel-collector/pkg/collector/pipeline.go

Line 167 in 6f38d97

p.ProcessReports(ctx, w, r)

This issue is comprised of 2 points:

• Consider exposing a logger interface to log errors within the pipeline class
• How can we catch/take action on ErrDropped, it doesn't seem to be bubbled up anywhere outside the class. ServeHTTP method calls it without looking for if there was an error and to my knowledge this doesn't get bubbled up to the HTTP server class (which does expose a logging interface)

I'm the first to admit i'm not the most advanced golang user, so let me know if i'm missing some implimentation detail that obviates the issue. I plan to impliment this at relatively large scale and knowing when we start dropping reports is going to be very valuable to log to our elastic stack.

Thanks!