Add more runtime.Memstats to reported metrics

[prateek]

Pulls in a few more https://golang.org/pkg/runtime/#MemStats fields:

• Lookups
• NextGC
• HeapObjects

[codecov]

Codecov Report

Merging #183 into master will decrease coverage by 0.55%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   78.62%   78.06%   -0.56%     
==========================================
  Files          86       86              
  Lines        3518     3524       +6     
==========================================
- Hits         2766     2751      -15     
- Misses        684      705      +21     
  Partials       68       68

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