Tempo: remove internal error reason for discarded spans

[joe-elliott]

What this PR does:
Removes the internal_error reason from tempo_discarded_spans_total. We are removing this reason for two reasons:

1. It's unnecessary. The sending process (otel collector, alloy, whatever) receives a gRPC error and is aware of any lost spans.
2. This metric was being incremented when the sending client cancelled context for any reason and stopped sending through no fault of Tempo's.

This change puts the responsibility of reporting failed sends on the sending process which has more context about why a send failed and can better log/metric the event.

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]