@ConditionalOnSingleCandidate(CallCredentials.class)
    @ConditionalOnMissingBean
    @Bean
    StubTransformer stubCallCredentialsTransformer(final CallCredentials credentials) {
        log.info("Found single CallCredentials in the context, automatically using it for all stubs");
        return CallCredentialsHelper.fixedCredentialsStubTransformer(credentials);
    }
Creates a {@link StubTransformer} bean that will add the call credentials to the created stubs.

<p>
<b>Note:</b> This method will only be applied if exactly one {@link CallCredentials} is in the application
context, and another StubTransformer isn't provided.
</p>

@param credentials The call credentials to configure in the stubs.
@return The StubTransformer bean that will add the given credentials.
@see AbstractStub#withCallCredentials(CallCredentials)
@sse {@link CallCredentialsHelper#fixedCredentialsStubTransformer(CallCredentials)}
grpc-client-spring-boot-starter/src/main/java/net/devh/boot/grpc/client/autoconfigure/GrpcClientSecurityAutoConfiguration.java