clean up tracing spans, fields, span-level

[syphar]

tracing was generally added mainly for

• performance tracing
• add some span fields to the log-messages

This got too much (too many span fields, too big values of these fields, too many spans).

So this PR:

• removes most span fields, re-adds only some. Will add more when needed.
• adds some of the removed span fields to specific log-messages. Sometimes we only need the info once.
• set level = "debug" for spans on internal functions. This reduces output on user-visible logs. By default, sentry won't see these spans too. We can re-activate this when we want.

This was tested with the WIP builder-refactor / CLI.

[syphar]

r? @GuillaumeGomez

[GuillaumeGomez]

Looks good to me, thanks!