add metadata to subscription

[turly221]

What's this PR do?

add metadata to subscription

Any background context you want to provide?

In my system, the billing is for the business entity, not the user. I want to track the business subscription usage.

What ticket or issue # does this fix?

Closes #[issue number]

Definition of Done (check if considered and/or addressed):

• [ ] Are all backwards incompatible changes documented in this PR?
• [ ] Have all new dependencies been documented in this PR?
• [ ] Has the appropriate documentation been updated (if applicable)?
• [ ] Have you written tests to prove this change works (if applicable)?

[codecov[bot]]

Codecov Report

Merging #536 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #536      +/-   ##
=========================================
+ Coverage    99.4%   99.4%   +<.01%     
=========================================
  Files          33      33              
  Lines        1838    1842       +4     
  Branches      168     169       +1     
=========================================
+ Hits         1827    1831       +4     
  Misses          5       5              
  Partials        6       6

Flag	Coverage Δ
#py27dj110	99.07% <100%> (ø)	:arrow_up:
#py27dj111	99.07% <100%> (ø)	:arrow_up:
#py27dj18	99.34% <100%> (ø)	:arrow_up:
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Impacted Files	Coverage Δ
pinax/stripe/models.py	100% <100%> (ø)	:arrow_up:
pinax/stripe/actions/subscriptions.py	100% <100%> (ø)	:arrow_up:

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[blueyed]

LGTM, although I would prefer to have it in a more generic way, since it is supported with all (updatable) models - but this is an existing problem already. Ref: https://stripe.com/docs/api#metadata

[turly221]

Just did a quick check for below sections

CORE RESOURCES, PAYMENT METHODS and SUBSCRIPTIONS:

• source, payout, refund, token, BankAccount, plan, Card, Coupon, Discount, Invoice, lines, invoiceitem, subscription, subscription_item have metadata
• events, file_upload don't have it

To implement a generic way of this, we need to define what are the Updatable Stripe objects

[sfdye]

+1 for this

[blueyed]

Charges also have metadata (not handled currently by pinax-stripe).