mpp_establish_conn related metrics is broken when enable async grpc server

[windtalker]

Bug Report

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1. Minimal reproduce step (Required)

When enable async grpc server, mpp_establish_conn related metrics(mpp_establish_conn_duration, mpp_establish_conn_count) is broken because in

tiflash/dbms/src/Flash/FlashService.cpp

Lines 256 to 260 in 731482e

	SCOPE_EXIT({
	GET_METRIC(tiflash_coprocessor_handling_request_count, type_mpp_establish_conn).Decrement();
	GET_METRIC(tiflash_coprocessor_request_duration_seconds, type_mpp_establish_conn).Observe(watch.elapsedSeconds());
	// TODO: update the value of metric tiflash_coprocessor_response_bytes.
	});

it is assumed that when AsyncFlashService::establishMPPConnectionAsync finishes, the establish connection also finishes. It is not true for async grpc server.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)