Add per-operation metric collector facility

[bartelink]

Adds the facility to supply a metrics collector at the individual operation level
This enables one to deterministically attribute metrics to individual calls on a given TableContext

[bartelink]

@samritchie re fsprojects#39 (comment), I did this as a spike to validate the effort/ripple effects with a view to using it in jet/equinox#321
Can post this as a PR on the main repo, but am happy for you to do it your own way in your own time too
Also can perhaps push some diffs and/or preparatory work (e.g. reformatting all member declaration lines with a max line length if 80/120 chars?) into fsprojects#40

[samritchie]

@bartelink I'll pull this branch down today & have a look at it, I'd like to put some tests around metricsCollector as well so I'll dig into what we get back from dynamodb-local and make sure it’s going to work.

[bartelink]

@samritchie Thanks - replaced with https://github.com/bartelink/FSharp.AWS.DynamoDB/pull/41