Spot termination exporter - fetch rebalance recommendations

[gjtempleton]

PR description from banzaicloud#15

What's in this PR?

• Adds instance_type label to terminationIndicator and terminationTime metrics to allow tracking of trends by instance type
• Adds monitoring for rebalance recommendation on instance metadata endpoint to add new metrics for when instances are being recommended for rebalance, includes the instance_id and instance_type labels

Why?

Rebalance recommendations provide potentially significant advance notice of potential spot disruption before the 2-minute notice of a spot termination. Adding the exposure of this metric allows cluster operators to gain advance warning of potentially large scale spot interruptions in their clusters.

The addition of the instance_type label also allows cluster operators to understand if their spot interruptions are clustered on specific instance types, leading to data driven consideration of instance type choices.