add LB policy metadata lookup API (#27307)

src/core/ext/filters/client_channel/client_channel.cc

@@ -2513,6 +2513,11 @@ class ClientChannel::LoadBalancedCall::Metadata
     return result;
   }
 
+  absl::optional<absl::string_view> Lookup(absl::string_view key,
+                                           std::string* buffer) const override {
+    return grpc_metadata_batch_get_value(batch_, key, buffer);
+  }
+
  private:
   LoadBalancedCall* lb_call_;
   grpc_metadata_batch* batch_;

src/core/ext/filters/client_channel/lb_policy.h

@@ -154,6 +154,9 @@ class LoadBalancingPolicy : public InternallyRefCounted<LoadBalancingPolicy> {
     /// Produce a vector of metadata key/value strings for tests.
     virtual std::vector<std::pair<std::string, std::string>>
     TestOnlyCopyToVector() = 0;
+
+    virtual absl::optional<absl::string_view> Lookup(
+        absl::string_view key, std::string* buffer) const = 0;
   };
 
   /// Arguments used when picking a subchannel for a call.