APM UI - Not able to search for child span/transaction labels

_Note: I've posted this on [Elastic Forum](https://discuss.elastic.co/t/apm-ui-not-able-to-search-for-child-span-transaction-labels/294081). But as it may also be a bug, I've posted this here too_

**Describe the bug**

I expected to be able to filter traces based on their transactions' labels. However as shown below I am not able to do that.

With the following distributed transaction:


When I see the child transaction from the "station-write" service, we can see that there is a "stationId" label:


When I see the parent transaction from the "gateway" service, we can see that there are other labels:


**To Reproduce**
Steps to reproduce the behavior:

Go to "Traces" page, enter in the search box the "labels.stationId" field(we can see that the auto complete works):

Filter by a given value, and see that it doesn't show any transactions

I couldn't find anywhere that said that it is(or is not) possible to search by labels in child spans/transactions. If it isn't possible to search by those labels, is there any way that I can add those labels to the parent transaction, in order for them to be searchable? The gateway service starts the trace, and this "station-write" service creates another transaction inside of that distributed trace(they are in different pods).

**Expected behavior**
I expected to be able to filter traces based on their transactions' labels.

**Versions**

**Kibana version**: 7.15.1
**Elasticsearch version**: 7.15.1
**APM Server version**: 7.15.1
**APM Agent language and version**: Node.JS at 3.26.0