avoid dynamic_cast to get Pi-tag for non_internal requests (#6868)

Optimization - See lots of stack traces spending cycles on dynamic_cast
for every request, when in reality, it's no longer needed as Http2ClientSession which
used to inherit PluginIdentity in the past is no longer doing that and
the client access protocol for access logs is being extracted directly
from the HttpSM context.

Author: sudheerv

proxy/ProxyTransaction.cc

@@ -43,10 +43,13 @@ ProxyTransaction::new_transaction(bool from_early_data)
   HttpTxnDebug("[%" PRId64 "] Starting transaction %d using sm [%" PRId64 "]", _proxy_ssn->connection_id(),
                _proxy_ssn->get_transact_count(), _sm->sm_id);
 
-  PluginIdentity *pi = dynamic_cast<PluginIdentity *>(this->get_netvc());
-  if (pi) {
-    _sm->plugin_tag = pi->getPluginTag();
-    _sm->plugin_id  = pi->getPluginId();
+  // PI tag valid only for internal requests
+  if (this->get_netvc()->get_is_internal_request()) {
+    PluginIdentity *pi = dynamic_cast<PluginIdentity *>(this->get_netvc());
+    if (pi) {
+      _sm->plugin_tag = pi->getPluginTag();
+      _sm->plugin_id  = pi->getPluginId();
+    }
   }
 
   this->increment_client_transactions_stat();