src/Zipkin/RealSpan.php
public function tag(string $key, string $value): void
    {
        $this->recorder->tag($this->traceContext, $key, $value);
    }
public function tag(string $key, string $value): void
    {
        $this->recorder->tag($this->traceContext, $key, $value);
    }
Tags give your span context for search, viewing and analysis. For example, a key
"your_app.version" would let you lookup spans by version. A tag {@link Zipkin\Tags\SQL_QUERY}
isn't searchable, but it can help in debugging when viewing a trace.

@param string $key Name used to lookup spans, such as "your_app.version". See {@link Zipkin\Tags} for
standard ones.
@param string $value, cannot be <code>null</code>.
@return void