Add JSON field path extraction for params/metrics (ticket 015)

RegexToMetricPrimitiveMatcher and the standalone param declaration
(DeclareParamRequest/DataValue::ParamDeclaration) gain an optional
json_field_path, an alternative to capture_group that reads a
dot-separated field path (e.g. "foo.bar") out of lines that parse as
JSON objects. It's additive and serde-defaulted, so existing
regex-based params/metrics are unaffected. Params.tsx now offers a
"JSON field" extraction option when the message match regex looks
like a JSON object.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/data_value.rs

@@ -8,12 +8,44 @@ use std::collections::BTreeMap;
 pub struct RegexToMetricPrimitiveMatcher {
     pub param_name: String,
     pub capture_group: String,
+    /// Alternative extraction source to `capture_group`: a dot-separated JSON field path (e.g.
+    /// `foo.bar`), read from lines that parse as a JSON object instead of via the regex capture
+    /// group. When present and non-empty, this takes precedence over `capture_group`.
+    /// `#[serde(default)]` so profile files/requests saved before this field existed still load
+    /// as plain regex-capture-group matchers.
+    #[serde(default)]
+    pub json_field_path: Option<String>,
     pub kind: MetricPrimitiveKind,
     /// Whether this param is a bucket label or its captured value (at most one Value-role param per metric); `#[serde(default)]` so profile files saved before this field existed still load as `Label`.
     #[serde(default)]
     pub role: ParamRole,
 }
 
+/// Reads a value out of a JSON object by dot-separated field path (e.g. `"foo.bar"`), for
+/// [`RegexToMetricPrimitiveMatcher::json_field_path`] and the sibling `json_field_path` on a
+/// standalone param declaration. Returns `None` when `text` doesn't parse as a JSON object, the
+/// path doesn't resolve to a value, or the resolved value isn't a scalar (string/number/bool) —
+/// there's no sensible flat text form of a nested object/array to feed into
+/// `parse_metric_primitive`.
+pub fn extract_json_field(text: &str, field_path: &str) -> Option<String> {
+    let root: serde_json::Value = serde_json::from_str(text).ok()?;
+    let mut current = root.as_object()?;
+    let mut segments = field_path.split('.').peekable();
+    loop {
+        let segment = segments.next()?;
+        let next_value = current.get(segment)?;
+        if segments.peek().is_none() {
+            return match next_value {
+                serde_json::Value::String(s) => Some(s.clone()),
+                serde_json::Value::Number(n) => Some(n.to_string()),
+                serde_json::Value::Bool(b) => Some(b.to_string()),
+                _ => None,
+            };
+        }
+        current = next_value.as_object()?;
+    }
+}
+
 /// See [`RegexToMetricPrimitiveMatcher::role`].
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
 #[serde(rename_all = "lowercase")]
@@ -45,6 +77,8 @@ pub enum DataValue {
     ParamDeclaration {
         message_match_regex: String,
         capture_group: String,
+        /// See `RegexToMetricPrimitiveMatcher::json_field_path`.
+        json_field_path: Option<String>,
         kind: MetricPrimitiveKind,
     },
     /// Paired with `IndexKey::MetricDeclaration`.