proposal: contrib/cloud.google.com/go/pubsub should support custom service names

[mickeyreiss]

It is useful to be able to configure span options, such as a default service name, but pubsubtrace.WrapReceiveHandler does not allow this override.

In particular, this signature:

func WrapReceiveHandler(s *pubsub.Subscription, f func(context.Context, *pubsub.Message)) func(context.Context, *pubsub.Message)

doesn't allow one to set the service name.

Maybe it should be something like this:

func WrapReceiveHandler(s *pubsub.Subscription, f func(context.Context, *pubsub.Message), , opts ...ReceiveOption) func(context.Context, *pubsub.Message)

Here's sql trace, for comparison, which does allow overrides via the third parameter on:

func Register(driverName string, driver driver.Driver, opts ...RegisterOption)

[knusbaum]

Enabling users to specify things like the service name is ideal.

I think 'WrapReceiveHandler` could just take a variadic list of StartSpanOption, and we can add those to a list of the existing options.

We do want to maintain backwards-compatibility, so we should give this function a new name, and have WrapReceiveHandler call the new function with no additional arguments.
Scratch that. A variadic function shouldn't require a rename since zero option arguments is valid. 😄