lc-observability 0.21.0

Pluggable observability sinks for LangChainRust.

The framework exposes the [MetricsSink] interface and an [ObsEvent] payload from lc-core (written by TokenTrackingLLM and AgentExecutor). This crate ships concrete sinks that implement it:

• [JsonLinesSink] (default jsonl feature): append each event as one JSON line to a file.
• [MongoSink] (mongodb feature): insert each event as a document.

Export failures are returned as [ObsError]; the framework logs a warn and the main flow continues — sinks never interrupt the agent.