tracing-subscriber: programmatically opt out of setting a global logger even when `tracing-log` is enabled

[lcmgh]

If one wants to opt-out of letting tracing-subscriber init. a global logger one has to disable the feature flag tracing-log.

There are setups when an app wants to decide by config whether a global logger should be set by tracing-subscriber. This can currently not be done because the setting must be configured on compile time.

Example scenarios that all should be configurable on runtime:

• Custom logger + Tracing as OTLP Exporter
• Tracing as logger AND OTLP exporter

To support both cases tracing-log must be set which then throws an error for the first scenario.

[sdtnjung]

Workaround: Use tracing-subscriber with default-features = false and only initialize the log_tracer when really needed.