feat: support metric custom column ordering and sub-setting

[nh13]

Fixes #177

[codecov]

Codecov Report

Attention: Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.20%. Comparing base (d574e5a) to head (b06b986).
Report is 2 commits behind head on main.

Files	Patch %	Lines
fgpyo/util/metric.py	91.30%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
- Coverage   89.21%   89.20%   -0.02%     
==========================================
  Files          18       18              
  Lines        2068     2084      +16     
  Branches      457      464       +7     
==========================================
+ Hits         1845     1859      +14     
- Misses        146      147       +1     
- Partials       77       78       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ameynert]

Additional test requested

[tfenne]

Could I suggest calling this something else - e.g. _fields_to_write()? My reasoning:

1. It's a little confusing to have header() and _header()
2. More importantly, I might think this would allow me to provide a custom header that doesn't match field names (e.g. [f.replace("_", " ") for f in in self.header()]

[tfenne]

Why is field_types optional? Given this is a function called by the Metric class itself, it feels like it should either always be provided, or be removed. How does the user who overrides this know whether or not they'll get a value?

[nh13]

@tfenne:

1. I've changed _header to _fields_to_write
2. This PR does not allow a customer header that doesn't match the field names, as we use the names returned from the header() method to look up attributes on the class (see the yield getattr(self, name) in the values() method)
3. I've made field_types not optional. Good catch, as this was a left-over from a previous attempt.

[ameynert]

Looks good!

[ameynert]

tick!