Enhance Visibility into Reactor FlatMap

[deleonenriqueta]

Description

A customer is experiencing a visibility gap in multiple Reactor Netty based applications that chain service calls using Mono.flatMap. Currently the reactor instrumentation only captures the first HTTP call, while subsequent calls within flatMap are labeled "No Deeper Visibility". They are requesting an enhancement to the Reactor instrumentation to ensure all HTTP calls within flatMap are instrumented.

Additional context

The customer has provided a repro and a POC to support troubleshooting and resolving this issue.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-378190

[deleonenriqueta]

The MonoFlatMap_Instrumentation class will be added to both the reactor-3.3.0 and netty-reactor-0.9.0 instrumentation modules. The reasoning for this can be found in the reactor-3.3.0 README.