[C++] Add OT spans for the scanner

[asfimport]

There should be one span for each "read batch" and one scan for each "read file". The batch spans should be children of the file span.

If possible it would be nice if the "read file" spans were children of the exec plan when the scanner is being used as a source node. I'm not sure if this is doable though since the scanner is often used outside the exec plan.

Reporter: Weston Pace / @westonpace
Assignee: Joost Hoozemans / @joosthooz

Related issues:

• [C++] Add rows scanned to open telemetry / profiling (blocks)

PRs and other links:

• GitHub Pull Request #11964
• GitHub Pull Request #12328
• GitHub Pull Request #12609

_{Note: This issue was originally created as ARROW-15067. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
Hmm, we could add a "parent span" or properly "parent span context" option to the scanner options. (That way you could link it up to a Flight span or something too.) You'd have to manually inject it to spans started in the scanner, though. (~2 lines of code or so for each span)

[asfimport]

David Li / @lidavidm:
Issue resolved by pull request 12609
#12609