startSpan(name, opts) {
		const r = {
			traceID: this.traceID,
			parentID: this.id,
			sampled: this.sampled,
			service: this.service
		};
		return this.tracer.startSpan(name, opts ? Object.assign(r, opts) : r);
	}
Start a child span.

@param {String} name
@param {Object?} opts
@returns {Span} Child span
@memberof Span