[APM] Fallback to terms agg search if terms enum doesn’t return results

[sorenlouv]

Problem
The suggestion component will currently use the terms enum API if no service names are given, and a normal search (using terms agg) otherwise. The terms enum approach is faster, and works well to provide initial results immediately.
However, when the user starts typing the terms enum API will only return terms that starts with the given search query. This means that the query "rum" won't return the term "opbeans-rum".

Solution
This PR solves this by falling back to ordinary search if terms enum doesn't provide any results. This strikes a balance between speed and accuracy.

Review without whitespace: https://github.com/elastic/kibana/pull/143619/files?diff=unified&w=1

[elasticmachine]

Pinging @elastic/apm-ui (Team:APM)

[kibana-ci]

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[gbamparop]

If I understand this correctly, it might be confusing to the user as the search strategy changes depending on whether there are data returned form the terms enum API so they will have a different experience in each case.

[sorenlouv]

If I understand this correctly, it might be confusing to the user as the search strategy changes depending on whether there are data returned form the terms enum API so they will have a different experience in each case.

You are right that it will change the search strategy on the fly now. But I think that actually creates the behaviour that users expect. Depending on the context, a component will currently either use terms enum or ordinary search. This means that sometimes the search query must match the start of the search term, and in other cases it can be a partial match in the middle. This is the bigger problem, that this attempts to solve.

What actual problems do you see this causing? It will only do the fall back if terms enum didn't have any results, so it's not like the user is any worse of.

[cauemarcondes]

LGTM

[sorenlouv]

@gbamparop and I discussed this offline and he is ok with the changes.