fix(spans): Do not trim essential fields

[jjbayer]

Since #3645, we're seeing incomplete or missing trace IDs on spans (see linked sentry issues). What happens is that after visiting a long span.description, there is no budget left for essential fields and the trimming processor drops or trims them.

• Introduce trim = "false" field attribute to disallow trimming of specific fields.
• Apply the new field to span.trace_id and other essential fields to prevent them from being partially scrubbed or dropped entirely.

Current limitations:

• Not trimming these fields leads to over-accepting bytes. This should not matter as long as the fields that have trim = "false" are limited in size (which they are currently not, see ref(protocol): Represent SpanId and TraceId as bytes #3535 as follow-up).
  • The more correct solution would be to subtract the length of trim = "false" fields before trimming any other fields.
  • Alternatively, we could decide to never over-accept, and drop items after visiting them if the budget is negative. I implemented this in 1136f76 but it resulted in more failing tests, because the current trimming processor already over-accepts a little bit (key lengths), and I did not want to touch the default behavior. cc @Dav1dde.

Fixes RELAY-2NY7, RELAY-2NY8, SNUBA-552.

[Dav1dde]

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