Add support for zero or more callbacks

[ocelotl]

As stated by the spec:

Zero or more callback functions.

[srikanthccv]

@ocelotl
Thinking about this again, Should this be "one or more callbacks" instead of "zero or more callbacks"? What does it mean to create an instrument with zero callbacks?

[ocelotl]

@ocelotl Thinking about this again, Should this be "one or more callbacks" instead of "zero or more callbacks"? What does it mean to create an instrument with zero callbacks?

It means we can instrument that does nothing 😅

It is ok to have an instrument with zero callbacks (the spec specifically states this), such instrument is harmless, but the real reason is that in the future we plan to add another way to register callbacks (#2454). With this feature a zero-callback instrument could receive a callback later.