Discovery Provider 0.3.12

Breaking changes for Service Providers

• Enforcement of required environment variables begins. List of variables and values can be found here https://github.com/AudiusProject/audius-protocol/wiki/Discovery-Provider-%E2%80%90-How-to-run#required-environment-variables

Full Changelog:
[ed48ab8] - Bump version to 0.3.12
[7a7b14e] - adding metadata to health_check.py routes (#536)
[0a22ba6] - API Signing for discovery provider (#532)