Previously, the fluentSink implementation was not thread-safe. On
configurations with multiple `loggerT` instances the fluentSink
interface would be shared between them and could cause panics at
runtime.

This commit adds a mutex to the fluentSink implementation that is locked
during output.

NB: httpSink is already thread safe by construction and interceptorSink
already has a mutex that manages concurrent access.

Resolves #81112

Release note: None