add metrics support

[John Gilbert]

• size of batch
• pipeline start and end timer
• eventType ratio
• other
• cw, datadog, other
• toPromise2 that returns metrics
• trace - xray?
• dynamodb capacity units?
• enable/disable via global/local options ?

[petermyers]

In addition to what you listed above:

• Lambda Xray integration
• New relic?
• Memory cache hits / misses
• Timing data - beyond just start and end timers for the pipeline, I'm thinking about enabling performance profiling. A utility you wrap pipeline steps in that adds additional timing data at a more granular level
• Pipeline ratios for batch - More so than just eventTypes since in practice a single event type might get processed by multiple pipelines