Support registering to gorm.DefaultCallback

[d4l3k]

Gorm currently has a data race when registering to multiple gorm databases at the same time. See https://github.com/jinzhu/gorm/issues/1916. It'd be nice if you could just directly register opentracing callbacks on gorm.DefaultCallback

[smacker]

Hm. Interesting! Thanks.

Though I'm not sure registering to DefaultCallback is the right way to do that. Also, I'm not sure this race is so dangerous. Registration should happen at the start of the application without concurrency involved.

But let me know if it IS a problem for you. Maybe we can figure something out without going against the recommended way to register callbacks.

[d4l3k]

I added a lock around the registration call so it's not an issue any more but still might be nice to have a solution. I run my tests with -race enabled so it breaks the CI build without any mitigation.

[d4l3k]

I also spin up multiple instances and services within my integration tests which means I have to have multiple databases.