Add a warning message for usage legacy OTLP HTTP endpoint (#4519)
Add a warning message for usage legacy OTLP HTTP endpoint (#4519)

Default OTLP HTTP endpoint 0.0.0.0:4318 enables legacy endpoint as well. It makes the migration painless, so we need to encourage users to update their configuration. This change adds a warning message in case if user has the legacy endpoint configured explicitly.
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector