Add concrete Langfuse tracer and prompt gateway

src/core/langfuse/LangfuseTracer.ts

@@ -0,0 +1,40 @@
+import { Langfuse } from 'langfuse';
+import { Tracer } from '../llm/Tracer';
+import type { GenerationHandle, GenerationInput, GenerationOutput, TraceHandle } from '../llm/Tracer';
+
+export class LangfuseTracer extends Tracer {
+  private readonly client: Langfuse;
+
+  constructor(opts: { publicKey: string; secretKey: string; baseUrl: string }) {
+    super();
+    this.client = new Langfuse({
+      publicKey: opts.publicKey,
+      secretKey: opts.secretKey,
+      baseUrl: opts.baseUrl,
+    });
+  }
+
+  startTrace(name: string, metadata?: Record<string, unknown>): TraceHandle {
+    const trace = this.client.trace({ name, metadata });
+    return {
+      id: trace.id,
+      startGeneration(input: GenerationInput): GenerationHandle {
+        const generation = trace.generation({
+          name: input.name,
+          model: input.model,
+          input: input.input,
+          metadata: input.metadata,
+        });
+        return {
+          end(output: GenerationOutput) {
+            generation.end({ output: output.output, usage: output.usage, metadata: output.metadata });
+          },
+        };
+      },
+    };
+  }
+
+  async flush(): Promise<void> {
+    await this.client.flushAsync();
+  }
+}