feat(outcomes): Add 'ignored' reason to client discard reasons list (#7635)

Adds a new discard reason `'ignored'` to the list of allowed reasons.
The intention is to use this new reason to primarily surface when spans
were ignored via the `ignoreSpans` deny list (an SDK option).
The naming choice is general on purpose to in the future also use this reason
with other telemetry type deny lists (e.g. ignoreErrors).

Author: Lms24

rust_snuba/src/processors/outcomes.rs

@@ -21,6 +21,8 @@ const CLIENT_DISCARD_REASONS: &[&str] = &[
     "cache_overflow",
     // an event was dropped by an event processor; may also be used for ignored exceptions / errors
     "event_processor",
+    // an event was dropped by an SDK ignore config (e.g. an `ignore_spans` deny list)
+    "ignored",
     // an event was dropped due to a lack of data in the event (eg: not enough samples in a profile)
     "insufficient_data",
     // an event was dropped due to an internal SDK error (eg: web worker crash)