@Nullable annotation is missing from methods that may return null

[m15o]

We have kotlin test codes using BraveTracer, but we get inaccurate warnings from the kotlin compiler.

For example, following code is warnde by the compiler with Unnecessary safe call on a non-null receiver of type Span message:

internal class TracerTest {
    @Test
    fun test() {
        val tracing = brave.Tracing.newBuilder().build()
        val tracer = BraveTracer(tracing.tracer(), BraveCurrentTraceContext(tracing.currentTraceContext()))
        assertThat(tracer.currentSpan()?.context()?.traceId()).isNull()
    }
}

This is because io.micrometer.tracing.brave.bridge package have @NonNullApi, but BraveTracer.currentSpan() do not have @Nullable annotation.
We are not sure, but other methods might also missing @Nullable annotation.

[marcingrzejszczak]

Yikes! Are you willing to file a PR to help us out?

[ttddyy]

But shouldn't it inherit @Nullable from the interface even though they are in different packages?
I see Tracer.currentSpan() interface is annotated with @Nullable.

[m15o]

By type castting BraveTracer to Tracer, the warning goes away.
I think BraveTracer has narrower type, so if you use BarveTracer itself, the BraveTracer.currnet() is treated as not-null.

[ttddyy]

Looking at Spring Framework, the implementation class also declares @Nullable if the method could return null.
For example, RestOperation interface against RestTemplate implementation class.

So, I think it would be a good PR for @m15o to fix this.