add prometheus middleware

[butonic]

This allows collecting call metrics for #2976

I'll add A PR to ocis that adds sth like this to all ocis services:

			"interceptors": map[string]interface{}{
				"prometheus": map[string]interface{}{
					"namespace": "ocis",
					"subsystem": "auth_basic",
				},
			},

it allows fetching metrics like this:

$ curl 127.0.0.1:9165/metrics -s | grep _requests | grep -v '#' | grep -v 'promhttp_metric_'
ocis_app_provider_grpc_requests_total 0
ocis_app_registry_grpc_requests_total 1
ocis_auth_basic_grpc_requests_total 0
ocis_auth_bearer_grpc_requests_total 0
ocis_auth_machine_grpc_requests_total 0
ocis_frontend_http_requests_total 0
ocis_gateway_grpc_requests_total 1
ocis_groups_grpc_requests_total 0
ocis_sharing_grpc_requests_total 0
ocis_storage_publiclink_grpc_requests_total 0
ocis_storage_shares_grpc_requests_total 0
ocis_storage_system_grpc_requests_total 6
ocis_storage_system_http_requests_total 0
ocis_storage_users_grpc_requests_total 0
ocis_users_grpc_requests_total 0

and after a single list received shares call with 500 accepted shares:

$ curl 127.0.0.1:9165/metrics -s | grep _requests | grep -v '#' | grep -v 'promhttp_metric_'
ocis_app_provider_grpc_requests_total 0
ocis_app_registry_grpc_requests_total 1
ocis_auth_basic_grpc_requests_total 1
ocis_auth_bearer_grpc_requests_total 0
ocis_auth_machine_grpc_requests_total 1
ocis_frontend_http_requests_total 1
ocis_gateway_grpc_requests_total 1013
ocis_groups_grpc_requests_total 0
ocis_sharing_grpc_requests_total 1
ocis_storage_publiclink_grpc_requests_total 0
ocis_storage_shares_grpc_requests_total 0
ocis_storage_system_grpc_requests_total 858
ocis_storage_system_http_requests_total 118
ocis_storage_users_grpc_requests_total 501
ocis_users_grpc_requests_total 5

This pr can easily be

• ported to master and
• used to compare master vs edge