Metrics

[frigus02]

Closes #39

This seems to be working. After running the example, metrics show up in AppInsights:

INSTRUMENTATION_KEY=??? cargo run --example metrics --features metrics

TODO:

• [x] more testing (e.g. does export of multiple values from ArrayAggregator work?)
• [ ] ~integrate into pipeline builder~ let's do that later
• [x] report some believable metrics in the example
• [x] documentation
• [x] can metrics be correlated to spans (requests or remote dependencies)? I can't see a way to get a traceid or spanid from a Record