Add 'eval' tag (#7209)

Add an "eval" tag to traced evaluation runs

Most of this PR is actually
https://github.com/hwchase17/langchain/pull/7207 but I can't diff off
two separate PRs

---------

Co-authored-by: Ankush Gola <9536492+agola11@users.noreply.github.com>

langchain/callbacks/tracers/evaluation.py

@@ -84,7 +84,7 @@ class EvaluatorCallbackHandler(BaseTracer):
         try:
             if self.project_name is None:
                 self.client.evaluate_run(run, evaluator)
-            with tracing_v2_enabled(project_name=self.project_name):
+            with tracing_v2_enabled(project_name=self.project_name, tags=["eval"]):
                 self.client.evaluate_run(run, evaluator)
         except Exception as e:
             logger.error(