[Logs UI] Retrieve log entry field values via the unified fields API

[weltenwort]

Summary

Right now the Logs UI is accessing the values of the log entry documents via _source, while some other parts of the UI (such as the query bar) use the mapping to determine the field list. This means that fields that don't have one-to-one correspondence between the mapping and the source will behave inconsistently. This is especially true for field types like alias fields and runtime fields. In order to make the behavior more consistent and support the various mapped field types the values of log entries should be accessed via the fields API.

Acceptance criteria

• The getLogEntries function of the logEntries lib uses the fields API instead of _source.
• The getLogItem function of the logEntries lib uses the fields API instead of _source.
• The functions to get examples for log entry anomalies and categories in the logAnalysis lib use the fields API instead if _source.

Open questions

• Does this depend on highlighting being based on the fields API as well? (Highlighting should leverage the new field retrieval API elasticsearch#59931)

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)