Implement Metrics SDK

[lzchen]

Implement Metrics SDK.

Issues:

• Remove absolute, monotonic metric instrument options #404
• Add observer, remove gauge #403 & Implement Metric Observer + Callbacks #178
• Implement "named" Meters in metrics #221
• Implement "record" for MeasureMetric #185
• Implement aggregators for metrics #413
• Rename handles to "bound instrument" #468
• Metrics: add possibility to release handles and observer instruments #435