Split metrics gathering from reporting

[danielmitterdorfer]

Currently there is no data model whatsoever for metrics but just plain log files. Each reporter just parses these log files and creates reports from them. We should define a dedicated metrics data model. Reporters should just be responsible for rendering these metrics.

Sub tasks:

• Define a data model. For now, we will just allow simple key, value pairs in the context of the triple (invocation_timestamp, track, track_setup).
• Implement a metrics store based on a dedicated Elasticsearch instance.
• Store metrics in the metrics store. This will be properly implemented in Move metrics to new profiler infrastructure #21.
• Use metrics store in reporting. In the first step, we will only use the metrics store for summary reports. Graphs will follow in Replace existing reporting graphics with Kibana #46.