fn filter(&self) -> EnvFilter {
        match self.opentelemetry_level {
            OpenTelemetryLevel::OFF => EnvFilter::new("off"),
            OpenTelemetryLevel::INFO => EnvFilter::new("info"),
            OpenTelemetryLevel::DEBUG => EnvFilter::new("debug"),
            OpenTelemetryLevel::TRACE => EnvFilter::new("trace"),
        }
    }
Returns a filter for opentelemetry layer.
It's applied to the layer only, but note that there might be a global filter applied to the
whole subscriber.
core/lib/vlog/src/opentelemetry/mod.rs