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/profile.rs

@@ -60,6 +60,10 @@ pub struct ParamDeclarationRecord {
     pub name: String,
     pub message_match_regex: String,
     pub capture_group: String,
+    /// See `crate::data_value::RegexToMetricPrimitiveMatcher::json_field_path`; `#[serde(default)]`
+    /// so profiles saved before this field existed still load as plain regex-capture-group params.
+    #[serde(default)]
+    pub json_field_path: Option<String>,
     pub kind: ProfileMetricPrimitiveKind,
 }
 
@@ -69,6 +73,7 @@ impl From<&DeclareParamRequest> for ParamDeclarationRecord {
             name: request.name.clone(),
             message_match_regex: request.message_match_regex.clone(),
             capture_group: request.capture_group.clone(),
+            json_field_path: request.json_field_path.clone(),
             kind: request.kind.into(),
         }
     }
@@ -80,6 +85,7 @@ impl From<&ParamDeclarationRecord> for DeclareParamRequest {
             name: record.name.clone(),
             message_match_regex: record.message_match_regex.clone(),
             capture_group: record.capture_group.clone(),
+            json_field_path: record.json_field_path.clone(),
             kind: record.kind.into(),
         }
     }
@@ -90,6 +96,10 @@ impl From<&ParamDeclarationRecord> for DeclareParamRequest {
 pub struct MetricParamRecord {
     pub param_name: String,
     pub capture_group: String,
+    /// See `crate::data_value::RegexToMetricPrimitiveMatcher::json_field_path`; `#[serde(default)]`
+    /// so profiles saved before this field existed still load as plain regex-capture-group matchers.
+    #[serde(default)]
+    pub json_field_path: Option<String>,
     pub kind: ProfileMetricPrimitiveKind,
     /// See `ParamRole`; `#[serde(default)]` so profiles saved before this field existed still load, defaulting to `Label`.
     #[serde(default)]
@@ -121,6 +131,7 @@ impl From<&DeclareMetricRequest> for MetricDeclarationRecord {
                 .map(|matcher| MetricParamRecord {
                     param_name: matcher.param_name.clone(),
                     capture_group: matcher.capture_group.clone(),
+                    json_field_path: matcher.json_field_path.clone(),
                     kind: matcher.kind.into(),
                     role: matcher.role,
                 })
@@ -144,6 +155,7 @@ impl From<&MetricDeclarationRecord> for DeclareMetricRequest {
                 .map(|param| RegexToMetricPrimitiveMatcher {
                     param_name: param.param_name.clone(),
                     capture_group: param.capture_group.clone(),
+                    json_field_path: param.json_field_path.clone(),
                     kind: param.kind.into(),
                     role: param.role,
                 })
@@ -626,6 +638,7 @@ mod tests {
             name: name.to_string(),
             message_match_regex: pattern.to_string(),
             capture_group: capture_group.to_string(),
+            json_field_path: None,
             kind: MetricPrimitiveKind::String,
         }
     }
@@ -639,6 +652,7 @@ mod tests {
             params: vec![RegexToMetricPrimitiveMatcher {
                 param_name: param_name.to_string(),
                 capture_group: capture_group.to_string(),
+                json_field_path: None,
                 kind: MetricPrimitiveKind::Number,
                 role: ParamRole::Label,
             }],