prometheus_tsdb_head_max_time and prometheus_tsdb_head_min_time values is milliseconds

[AndorCS]

Proposal

Use case. Why is this important?
Since prometheus use standard values in seconds, having prometheus_tsdb_head_max_time and prometheus_tsdb_head_min_time in milliseconds is a bit confusing.

Bug Report

What did you do?
Query prometheus_tsdb_head_max_time - prometheus_tsdb_head_min_time
What did you expect to see?
1339.077
What did you see instead? Under which circumstances?
1339077
Environment
any

• System information:
  any
• Prometheus version:

prometheus_build_info{branch="HEAD",goversion="go1.11.1",revision="167a4b4e73a8eca8df648d2d2043e21bdb9a7449",version="2.4.3"}

• Alertmanager version:
  N/A
• Prometheus configuration file:
  N/A
• Alertmanager configuration file:
  N/A
• Logs:
  N/A

[simonpasquier]

The root issue is prometheus/tsdb#395. @krasi-georgiev @gouthamve this isn't the first report we've got about the TSDB metrics not being consistent with the best practices. Personally I'd go with what @SuperQ suggested here.