fix: export subscriber type define

[qz1543706741]

please help merge #183
@johnprg @stevenh @mme @suhasdeshpande

[Cyronius]

agreed -- AgentSubscriber needs to be exported from the index file as in the PR #183

It shows it being imported and used in the documentation, so I suspect this is just an oversight. My workaround has been to define the types for the callbacks myself until this fix is in place.

[derekwsgray]

There's a typo (,) in that URL to the PR. But please can we get this in? We too have to redefine the types ourselves.

[qz1543706741]

There's a typo (,) in that URL to the PR. But please can we get this in? We too have to redefine the types ourselves.

fixed it