Splunk RUM browser agent doesn't register any context propagators as capturing traceparent
from server-timing
headers is done by extending existing OpenTelemetry instrumentations. However, you can register a context propagators by using OpenTelemetry API:
import {propagation} from '@opentelemetry/api';
import {B3Propagator} from '@opentelemetry/propagator-b3';
propagation.setGlobalPropagator(new B3Propagator());
Notice that when using OpenTelemetry API directly make sure the API version you're using matches the one used by Splunk RUM Agent.