Consider introducing a --metadata option

[kemzeb]

Describe the feature

Given #391, #386, and the --license option that was introduced some time ago, there is a clear desire to provide more package metadata and aggregate data in order to get useful insights from the environment.

I propose that we introduce a metadata option that can a predefined value e.g.:

pipdeptree --metadata license # or -m license

Which leads me to consider if all rendering options should allow users to pass more than 1 metadata value e.g.:

pipdeptree --metadata size,license,keyword

I can see this being useful for machine-readable formats (e.g. json), but for rendering options like text it would make the dependency tree very hard to read.

We could try to:

• Throw all the possible metadata we would support into groups and determine if a rendering option supports rendering that group (or this could instead be done on a per-metadata-value basis e.g. should text support Home-page metadata)
• ... and (maybe?) restrict the number of metadata values that the rendering options can include, e.g.:

pipdeptree --metadata license,size
# Exit with an error here since the text render can only render one metadata value (to maintain readability)

We would need to somehow communicate this design in the help message so that users won't be surprised by certain input. Maybe a combination of the --metadata description and the main description would be a solution to that.

[stasfilin]

I can pick for development if this is still relevant

[kemzeb]

I was planning on tackling this myself eventually, but if you have the time feel free to do so!