Default metrics file

[dberenbaum]

Goal: make it easier to use experiment tracking in DVC (especially exp show) without worrying about pipelines, data management, etc.

DVC already has a default params.yaml that does not need to be specified in any stage for parameters to be tracked. Having similar defaults for metrics (for example, metrics.json) could enable DVC to read these artifacts without needing to specify them in dvc.yaml.

Example:

$ git init
$ dvc init
$ echo '{"acc": 0.5}' > metrics.json
$ git add .
$ git commit -m "adds metrics"
$ echo '{"acc": 0.6}' > metrics.json
$ dvc exp show
┏━━━━━━━━━━━━┳━━━━━━━━━━┳━━━━━━━┓
┃ Experiment ┃ Created  ┃ acc   ┃
┡━━━━━━━━━━━━╇━━━━━━━━━━╇━━━━━━━┩
│ workspace  │ -        │ 0.6   │
│ main       │ 04:43 PM │ 0.5   │
│ 8968a78    │ 04:43 PM │ 0.5   │
└────────────┴──────────┴───────┘

This would allow for users to get started quickly with experiment tracking by writing out their metrics to the specified locations without needing any other dvc-specific configuration.

[dberenbaum]

Closing in favor of #8371