Support context extract.

[wu-sheng]

let reporter = MockReporter::new();
        let mut context = TracingContext::new(&reporter).unwrap();
        let span1 = context.create_entry_span(String::from("op1"), None, &MockerHeader {});
        {
            assert_eq!(span1.span_id(), 0);
            let mut span2 = context.create_local_span(String::from("op2"), Some(&span1));
            span2.tag(Tag::new(String::from("tag1"), String::from("value1")));
            {
                assert_eq!(span2.span_id(), 1);
                let mut span3 = context.create_local_span(String::from("op3"), Some(&span2));
                assert_eq!(span3.span_id(), 2);

                context.finish_span(span3);
            }
            context.finish_span(span2);
        }
        context.finish_span(span1);

The header carrier is

    struct MockerHeader {}

    impl Extractable for MockerHeader {
        fn extract(&self, key: String) -> &str {
            "1-My40LjU=-MS4yLjM=-4-1-1-IzEyNy4wLjAuMTo4MDgw-Iy9wb3J0YWw=-MTIz"
        }
    }