[APM] Add message fields to metadata table

[formgeist]

Summary

Related to the Java agent implementing messaging data (elastic/apm#143), we want to add those fields to the metadata tab (when available).

Field names in elasticsearch:

• transaction.message
• span.message

Solution

• Add new Message section to the metadata table for Transactions and Spans
  • Transactions: Add it between Process and Agent sections https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/apm/public/components/shared/MetadataTable/TransactionMetadata/sections.ts#L7
  • Spans: Add it between Service and Agent sections https://github.com/elastic/kibana/blob/master/x-pack/legacy/plugins/apm/public/components/shared/MetadataTable/SpanMetadata/sections.ts#L17
• For context.message.age.ms, we expect the calculation to be an estimate more than an accurate measurement; therefore we need to treat 0 and possibly surrounding values (negative and positive) as ~0. Please refer to the original comment in the meta issue. (postponed for the first iteration)

[elasticmachine]

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

[sorenlouv]

Blocked until elastic/apm-server#2697 is implemented

[formgeist]

@alvarolobato Just a heads up; we have this implementation issue re: the message fields from the Java agent. I added this to the Observability roadmap spreadsheet too.

[formgeist]

@sqren Although it's currently blocked waiting for server changes, this is targeted for 7.6 so I've added that label and put it in the lane for 7.6 planned items.

[sorenlouv]

Thanks @formgeist

[cauemarcondes]

@sqren @formgeist I think this issue is not blocked, as we can still add those fields to the metadata table, the only point is that they would only show up after the Java agent is implemented. And if one of them is required, the empty message will show instead.

[formgeist]

Looks like the PR is merged so it's unblocked elastic/apm-agent-java#906

[sorenlouv]

@formgeist Great that java support has landed! Although think we'll need server support as well before the UI can start consuming this elastic/apm-server#2697

@cauemarcondes True, we could start without data. Historically we've just seen how implementing features without proper data is more error prone and takes more time. So generally we try to postpone implementation until the data is available.

[formgeist]

There's a new field being added to the message context, so I've updated the description.

[eyalkoren]

There is an intention to use context.message.age.ms for doing some aggregation and add it to the Trace metadata, and this part was postponed to a later step.
However, this field may be useful as is in the span/transaction span details, so worth adding it in the first iteration without any special handling.

[sorenlouv]

@eyalkoren we'll show everything inside message (so also age.ms). @cauemarcondes already implemented this in #54017 (comment)