Support for multiple CoWebFilters that change CoroutineContext

[ilya40umov]

This has been brought up here: #27522 (comment) and here micrometer-metrics/tracing#174 (comment).

Problem: if multiple CoWebFilters are called in a chain, the coroutine context from the last one will end up saved in CoWebFilter.COROUTINE_CONTEXT_ATTRIBUTE. Additionally, each subsequent filter won't be able to take advantage of context modifications from the previous filter in the chain.

E.g. if I have the following two filters in my project:

• CorrIdFilter.kt
• ObservationFilter
  only one of them will be able to provide coroutine context for the actual controller method.

Proposed solution: check if attribute is already present in the context and re-use it within the filter. Here is a fast-and-dirty implementation: ImprovedCoWebFilter