Update README, node level _lifetime metric types

[VCCPlindsten]

• Update README.md with new metrics, metric types
• Correct node level _lifetime metrics to Gauge (they are an average over the lifetime of the process)

[VCCPlindsten]

Previous update to master was somewhat incomplete.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.04%. Comparing base (119ace0) to head (0a126dc). Report is 144 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #374 +/- ## ========================================== - Coverage 93.60% 93.04% -0.56% ========================================== Files 15 15 Lines 641 633 -8 ========================================== - Hits 600 589 -11 - Misses 41 44 +3 ```

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