Add support for timing breakdowns

[felixbarny]

Implementation of agent breakdown metrics (elastic/apm#78)

Checklist/TL;DR version of the design:

• Calculate self-time of transactions and spans
  • Prerequisite: Spans have a reference to their parent
  • Add a child duration reentrant timer to transactions and spans
• Add feature flag
• Metrics registry enhancements
  • Add support for timers
  • Limit the max amount of timers
  • Add support for counters
  • Optional: guard against concurrent reads/writes
• Track span self-times on the transaction
  • Prerequisite: Spans have a reference to their transaction
  • On span end:
    • track the span self-time on the corresponding transaction
  • On transaction end
    • track all self-times on the metric registry
    • track the transaction's self time with the span.type app
    • track the transaction's duration on the metrics registry
• Reporting metrics
  • Add support for top-level labels for transaction.name, transaction.type, span.type and span.subtype
  • Reset counters for Timers and the transaction.breakdown.count counter every time the metrics are reported
• Acceptance tests
• Evaluate the performance impact
• Document metrics