Metric card template

Adding a metric card template, based on ideas and edits from @sashavor and I, as well as from comments from @lhoestq and others (thank you!).

All feedback is welcome, but am especially curious about feedback in terms of:
- things that should be included but aren't
- things that are included but should be changed or removed
- the instructions I included, and whether they should be added to, clarified, or deleted altogether

Comments:
_The documentation is not available anymore as the PR was closed or merged._

Looks like a great start! I have a general comment and a few specific comments.

My general comment is I wonder if we need a post for this template and the data and model card templates (or a combined one?) explaining why this documentation is needed and how it serves both the writer and the audience.

Specific comments:
- Maybe we can add some more desiderata to the overview instructions like: what task was the metric originally developed for, what tasks is it used for now, what is the range of possible outputs?
- In the data card, we call the data instances inputs `fields`. It might be good to synchronize on that across the templates and change `input_name` to `input_field`? Also are the instructions for the `input_name` complete? It ends with 'In the *' and I'm not sure what that refers to.
- 'Values' seems ambiguous to me, maybe 'scores' would be more explicit? Also could add a request for the range of possible outputs.
- We could add a reference in the examples section to the overview section if that's where further explanation should go. Suggestion to add: 'Provide a range of examples that show both typical and atypical results' or something similar.
- I'm not sure if we'd want to add this to the example section or make a new section, but it would be good to prompt somewhere for links to specific use cases in HF
- In the limitations and bias section, add 'with links'


Looks like a great start! I have a general comment and a few specific comments.

My general comment is I wonder if we need a post for this template and the data and model card templates (or a combined one?) explaining why this documentation is needed and how it serves both the writer and the audience.

Specific comments:
- Maybe we can add some more desiderata to the overview instructions like: what task was the metric originally developed for, what tasks is it used for now, what is the range of possible outputs?
- In the data card, we call the data instances `fields`. It might be good to synchronize on that across the templates and change `input_name` to `input_field`? Also are the instructions for the `input_name` complete? It ends with 'In the *' and I'm not sure what that refers to.
- 'Values' seems ambiguous to me, maybe 'scores' would be more explicit? Also could add a request for the range of possible outputs.
- We could add a reference to the examples section to the overview section if that's where further explanation should go. Suggestion to add: 'Provide a range of examples that show both typical and atypical results' or something similar.
- I'm not sure if we'd want to add this to the example section or make a new section, but it would be good to prompt somewhere for links to specific use cases in HF
- In the limitations and bias section, add 'with links'


Thanks for your feedback, @mcmillanmajora ! I totally agree that we should write a post -- we were going to write one up when we are done with a good chunk of the metric cards, but we can also do that earlier :smile: 

With regards to your more specific comments:

- It is our intention to put what the metric was developed for (whether it is a specific task or dataset, for example). You can see the [WER](https://github.com/huggingface/datasets/tree/master/metrics/wer) metric card for that.
- `input_field` works for me!
- the values aren't always scores, it's more like the values the metric can take. And it does include the range of possible values, including the max and min, that are outputted.
- I like the suggestion to add: 'Provide a range of examples that show both typical and atypical results' :hugs: 
- I have been putting specific use cases in 'Further references', just because there isn't always something to put there, especially for less popular metrics

Oh cool! I was just looking at the template, it definitely helps seeing an example metric card. Based on just the instructions, I had assumed that examples meant research papers where the metric was used to evaluate a model, but I like the explicit coding examples! 

Oh cool! I was just looking at the template, it definitely helps seeing an example metric card. Based on just the instructions, I had assumed that examples meant research papers where the metric was used to evaluate a model, but I like the explicit coding examples!