Change Duration unit return type to integer

[jonrandahl]

Based on current conversations with @andrew-pickin-epi, et al, the duration value returned from the gem's error control needs to be the expected integer type to resolve issues with the current tooling.

Also noted in these tickets:

• https://github.com/epimorphics/hmlr-linked-data/issues/111
• https://github.com/epimorphics/hmlr-linked-data/issues/104

Definition of done:

• [x] Resolve the elapsed value to be in microseconds and presented as an integer

[tomguilbert]

@jonrandahl during some ticket gardening we spotted that this looks like a duplicate of https://github.com/epimorphics/json-rails-logger/issues/41 - is that true and do you need both?

[jonrandahl]

Finally finished!