Add StreamingHub metrics

[mayuki]

This PR introduces metrics related to StreamingHub using System.Diagnostics.Metrics.

• see: https://learn.microsoft.com/en-us/dotnet/core/diagnostics/metrics

Meter: MagicOnion.Server

Metric	Unit	Tags
magiconion.server.streaminghub.connections	{connection}	rpc.system, rpc.service
magiconion.server.streaminghub.method_duration	ms	rpc.system, rpc.service, rpc.method
magiconion.server.streaminghub.method_completed	{request}	rpc.system, rpc.service, rpc.method, magiconion.streaminghub.is_error
magiconion.server.streaminghub.exceptions	{exception}	rpc.system, rpc.service, rpc.method, error.type

Tags

Tag name	Value
rpc.system	magiconion
rpc.service	StreamingHub interface name (e.g. IGreeterService)
rpc.method	StreamingHub method name (e.g. HelloAsync)
magiconion.streaminghub.is_error	Whether a StreamingHub method call succeeded or failed. (e.g. true or false)
error.type	Thrown exception type (e.g. System.InvalidOperationException)