ref(server): Refactor get_transaction_name in metric extraction (#2405)

Internal refactor that separates tracking a statsd metric for
unparameterized transaction names from the logic of extracting a
transaction name from the event. This will help with moving this
functionality to `FieldValueProvider`.

relay-server/src/metrics_extraction/transactions/mod.rs

@@ -22,6 +22,10 @@ use relay_general::store::utils::{
 
 pub mod types;
 
+/// Placeholder for transaction names in metrics that is used when SDKs are likely sending high
+/// cardinality data such as raw URLs.
+const PLACEHOLDER_UNPARAMETERIZED: &str = "<< unparameterized >>";
+
 /// Extract transaction status, defaulting to [`SpanStatus::Unknown`].
 /// Must be consistent with `process_trace_context` in [`relay_general::store`].
 fn extract_transaction_status(trace_context: &TraceContext) -> SpanStatus {
@@ -55,11 +59,7 @@ fn extract_geo_country_code(event: &Event) -> Option<String> {
  geo.country_code.value().cloned()
 }
 
-fn is_low_cardinality(source: Option<&TransactionSource>) -> bool {
- // `None` is used to mark a legacy SDK that does not send the transaction name,
- // and we assume sends high-cardinality data. See `is_high_cardinality_transaction`.
- let Some(source) = source else { return false };
-
+fn is_low_cardinality(source: &TransactionSource) -> bool {
  match source {
  // For now, we hope that custom transaction names set by users are low-cardinality.
  TransactionSource::Custom => true,
@@ -91,36 +91,25 @@ fn is_low_cardinality(source: Option<&TransactionSource>) -> bool {
 /// High-cardinality sources are excluded to protect our metrics infrastructure.
 /// Note that this will produce a discrepancy between metrics and raw transaction data.
 fn get_transaction_name(event: &Event) -> Option<String> {
- let original_transaction_name = match event.transaction.value() {
- Some(name) => name,
- None => {
- return None;
- }
- };
+ let original = event.transaction.value()?;
 
  let source = event
  .transaction_info
  .value()
  .and_then(|info| info.source.value());
 
- let use_original_name = is_low_cardinality(source);
-
- let name_used;
- let name = if use_original_name {
- name_used = "original";
- Some(original_transaction_name.clone())
- } else {
- // Pick a sentinel based on the transaction source:
- match source {
- None | Some(TransactionSource::Other(_)) => {
- name_used = "none";
- None
- }
- _ => {
- name_used = "placeholder";
- Some("<< unparameterized >>".to_owned())
- }
- }
+ match source {
+ Some(source) if is_low_cardinality(source) => Some(original.clone()),
+ Some(TransactionSource::Other(_)) | None => None,
+ Some(_) => Some(PLACEHOLDER_UNPARAMETERIZED.to_owned()),
+ }
+}
+
+fn track_transaction_name_stats(event: &Event) {
+ let name_used = match get_transaction_name(event).as_deref() {
+ Some(self::PLACEHOLDER_UNPARAMETERIZED) => "placeholder",
+ Some(_) => "original",
+ None => "none",
  };
 
  relay_statsd::metric!(
@@ -129,13 +118,10 @@ fn get_transaction_name(event: &Event) -> Option<String> {
  sdk_name = event
  .client_sdk
  .value()
- .and_then(|c| c.name.value())
- .map(std::string::String::as_str)
+ .and_then(|sdk| sdk.name.as_str())
  .unwrap_or_default(),
  name_used = name_used,
  );
-
- name
 }
 
 /// These are the tags that are added to all extracted metrics.
@@ -274,6 +260,7 @@ impl TransactionExtractor<'_> {
  return Err(ExtractMetricsError::InvalidTimestamp);
  }
 
+ track_transaction_name_stats(event);
  let tags = extract_universal_tags(event, self.config);
 
  // Measurements