Notify channel from user options is silently overridden in connection pool

[tulzke]

Description

In go-tarantool, there is an option to pass a Notify channel in connection options to receive connection-related events (connect / disconnect, etc.).
This is useful, for example, to build client-side metrics based on connection lifecycle events.

Relevant code

Notify option handling in single connection:
https://github.com/tarantool/go-tarantool/blob/v2.4.1/connection.go#L323

This mechanism works correctly when using a direct Connection.
However, after migrating to connection_pool, I noticed that all connection-related metrics based on Notify stopped working.

During debugging, I found the following behavior:

• Events are still being written, but not into the user-provided Notify channel.
• On the receiving side, the user-provided channel remains silent (no events).
  The root cause appears to be that the pool implementation overrides the Notify channel provided by the user.

Specifically

In connection_pool.go, the Notify field from user options is replaced with an internally created channel:
https://github.com/tarantool/go-tarantool/blob/v2.4.1/pool/connection_pool.go#L1309

This internal channel is created inside the pool:
https://github.com/tarantool/go-tarantool/blob/v2.4.1/pool/connection_pool.go#L143

As a result

• The original user-provided Notify channel loses all writers.
• The newly created internal Notify channel has no readers outside the pool.
• Connection events are effectively lost from the user’s perspective.

This behavior is not obvious from the API and silently breaks setups that rely on Notify for observability when switching from a single connection to a pool.

Expected behavior

One of the following (or an explicit documented alternative):

• Preserve and forward events to the user-provided Notify channel.
• Fan-out events to both internal and user-provided channels.
• Or clearly document that Notify is ignored / replaced when using connection_pool, and provide another supported way to observe connection events.

At the moment, the current behavior leads to lost events without any warning.