Add pendingResponses to ServerMetrics

[minwoox]

Currently, pendingResponses are managed in GracefulShutdownSupport. It would be beneficial to manage all server-related metrics within ServerMetrics, which will be added in #5627.

armeria/core/src/main/java/com/linecorp/armeria/server/Server.java

Lines 581 to 582 in 8678567

	meterRegistry.gauge("armeria.server.pending.responses", gracefulShutdownSupport,
	GracefulShutdownSupport::pendingResponses);

Side note: the difference between activeRequests and pendingResponses is that the pendingResponses does not include the request count for transient services.