Incomplete support for the Tag interface

[craig-day]

It seems like the OTSpan implementation doesn't fully support the Tag interface for adding tags of custom types to a span. Specifically, Tag#set is not called to allow a tag implementation to add additional tags to a span aside from the initial value.

Tag#getKey() is called here and then the tag implementation is lost and seems to not be used again. https://github.com/DataDog/dd-trace-java/blob/78bfb6f2c4de93acf7049522befe27413fd5d87c/dd-trace-ot/src/main/java/datadog/opentracing/OTSpan.java#L89-L93

For reference, if we look at opentracing's implementation of MockSpan, they use Tag#set and pass in this for the span instance so the tag implementation can appropriately tag the span.

    @Override
    public <T> MockSpan setTag(Tag<T> tag, T value) {
        tag.set(this, value);
        return this;
    }

A specific use-case: I want to have a MapTag implementation. This implementation would iterate the key-value pairs of the map and check the value types and act accordingly. This would allow me to recursively descend a map by calling Span#setTag from within Tag#set and pass it another MapTag with a composite key. Below is an example implementation that should work, but does not because it never gets called.

public final class MapTag implements Tag<Map<String, Object>> {

  private final String prefix;
  
  public MapTag(String prefix) {
    this.prefix = prefix;
  }

  @Override
  public String getKey() {
    return this.prefix;
  }

  @Override
  public void set(Span span, Map<String, Object> map) {
    map.forEach((k, v) -> {
      String compositeKey = this.prefix + "." + k;

      if (v instanceof Boolean b) {
        span.setTag(compositeKey, b);
      } else if (v instanceof Number n) {
        span.setTag(compositeKey, n);
      } else if (v instanceof Map<?, ?> m) {
        span.setTag(new MapTag(compositeKey), (Map<String, Object>) m);
      } else if (v != null) {
        span.setTag(compositeKey, v.toString());
      }
    });
  }
}