Output evaluation statistics

[danielfullmer]

It would be nice if nix-eval-jobs could also output evaluation statistics in the JSON output for each derivation, similar to what we get from nix-build when setting NIX_SHOW_STATS=1:

$ NIX_SHOW_STATS=1 nix-build . -A hello
{
  "cpuTime": 0.07112400233745575,
  "envs": {
    "bytes": 149776,
    "elements": 8144,
    "number": 5289
  },
  "gc": {
    "heapSize": 402915328,
    "totalBytes": 7441872
  },
  "list": {
    "bytes": 21416,
    "concats": 55,
    "elements": 2677
  },
  "nrAvoided": 7500,
  "nrFunctionCalls": 4661,
  "nrLookups": 1970,
  "nrOpUpdateValuesCopied": 283430,
  "nrOpUpdates": 448,
  "nrPrimOpCalls": 3363,
  "nrThunks": 43562,
  "sets": {
    "bytes": 5303360,
    "elements": 329564,
    "number": 1896
  },
  "sizes": {
    "Attr": 16,
    "Bindings": 16,
    "Env": 16,
    "Value": 24
  },
  "symbols": {
    "bytes": 203048,
    "number": 21009
  },
  "values": {
    "bytes": 1350384,
    "number": 56266
  }
}
this path will be fetched (0.05 MiB download, 0.22 MiB unpacked):
  /nix/store/g2m8kfw7kpgpph05v2fxcx4d5an09hl3-hello-2.12.1
copying path '/nix/store/g2m8kfw7kpgpph05v2fxcx4d5an09hl3-hello-2.12.1' from 'https://cache.nixos.org'...
/nix/store/g2m8kfw7kpgpph05v2fxcx4d5an09hl3-hello-2.12.1

[Mic92]

The question is though when to print this. nix-eval-jobs has several background workers each with their own statistics and workers are restarted if they consume too much memory. This makes just using evaluation statistics provided by nix itself hard to use.