OpenTelemetry support enhancement

[cescoffier]

In order to improve the OpenTelemetry tracing support in reactive messaging we need:

1. Introduce an SPI that allows decorating messages emitted by emitters. This SPI would be called on the same thread as the one calling emitter.send(...), and the SPI implementor should be able to extract the current OTel Context and create the TracingMetadata. This metadata will be added to the emitted message.
2. When a message is associated with a duplicated context (the message could be created from an emitter or from an inbound connector), an SPI should be created to extract the potential TracingMetadata and attach it to the duplicated context. This would allow OTel to re-attach.

In terms of ITs, there are already existing tracing-related IT, which verify the propagation along a pipeline and within a single protocol (like Kafka).
In addition to this, we need to check that:

• when emitting a message using an emitter, existing tracing details are propagated
• when emitting a payload using an emitter, existing tracing details are propagated
• when receiving a message from a protocol supporting tracing, the tracing details are propagated and associated with the duplicated context
• OTel can re-attach from the duplicated context

\CC @radcortez @brunobat @ozangunalp

[cescoffier]

@ozangunalp @radcortez, where are we on this one?

[radcortez]

I believe that #1678 takes care of this.

[ozangunalp]

Closing this as #1678 takes care of it.