Add BillingPlanFeatureValueType Enum, Invalid_Metric Errors

A new enum type BillingPlanFeatureValueType (currently only ADD_ON) is introduced to describe optional feature value types for billing plans. The BillingPlanFeatureValue type now includes an optional valueType field that references this enum, letting you specify if a feature value is an add‑on. Two analytics target errors have been expanded with the INVALID_METRIC value, affecting both create and update mutation error codes. Clients should adjust error‑handling logic to listen for AnalyticsTargetCreateUserErrorCode.INVALID_METRIC and AnalyticsTargetUpdateUserErrorCode.INVALID_METRIC. Existing clients that deconstruct BillingPlanFeatureValue objects will need to optionally read valueType but can continue ignoring it if absent. Keep an eye on future revisions; BillingPlanFeatureValueType may receive additional values beyond ADD_ON.