This allows integrations of Sleuth to use the same approach for 2.2.x as 3.x:
If you have a custom base propagation format, override the `BaggagePropagation.Factory`
bean instead of `ExtraFieldsPropagation.Factory`
Note: one subtle difference 2.2.x to 3.x is the change in the primary inject format.
2.2.x is
```java
return BaggagePropagation.newFactoryBuilder(B3Propagation.newFactoryBuilder()
.injectFormat(B3Propagation.Format.MULTI).build());
```
3.0 is
```java
return BaggagePropagation.newFactoryBuilder(B3Propagation.newFactoryBuilder()
.injectFormat(B3Propagation.Format.SINGLE_NO_PARENT).build());
```
per #1607
See https://github.com/spring-cloud/spring-cloud-gcp/issues/2268
e19e0ca169