Return error if span context was not found (grafana#11207)

**What this PR does / why we need it**:
This is a follow up to grafana#10956.

`lokigrpc.GetParentSpanForRequest` would ignore
`opentracing.ErrSpanContextNotFound`. However, that should be up to the
caller to decide whether this error should be ignored or not.

**Checklist**
- [ ] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](grafana@d10549e)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](grafana@0d4416a)

pkg/scheduler/scheduler.go

@@ -333,7 +333,7 @@ func (s *Scheduler) enqueueRequest(frontendContext context.Context, frontendAddr
 	// information, since that is a long-running request.
 	tracer := opentracing.GlobalTracer()
 	parentSpanContext, err := lokigrpc.GetParentSpanForRequest(tracer, msg)
-	if err != nil {
+	if err != nil && err != opentracing.ErrSpanContextNotFound {
 		return err
 	}
 

pkg/util/httpgrpc/carrier.go

@@ -39,11 +39,7 @@ func GetParentSpanForHTTPRequest(tracer opentracing.Tracer, req *weaveworks_http
 	}
 
 	carrier := (*HeadersCarrier)(req)
-	extracted, err := tracer.Extract(opentracing.HTTPHeaders, carrier)
-	if err == opentracing.ErrSpanContextNotFound {
-		err = nil
-	}
-	return extracted, err
+	return tracer.Extract(opentracing.HTTPHeaders, carrier)
 }
 
 func GetParentSpanForQueryRequest(tracer opentracing.Tracer, req *queryrange.QueryRequest) (opentracing.SpanContext, error) {
@@ -52,11 +48,7 @@ func GetParentSpanForQueryRequest(tracer opentracing.Tracer, req *queryrange.Que
 	}
 
 	carrier := opentracing.TextMapCarrier(req.Metadata)
-	extracted, err := tracer.Extract(opentracing.TextMap, carrier)
-	if err == opentracing.ErrSpanContextNotFound {
-		err = nil
-	}
-	return extracted, err
+	return tracer.Extract(opentracing.TextMap, carrier)
 }
 
 func GetParentSpanForRequest(tracer opentracing.Tracer, req Request) (opentracing.SpanContext, error) {