add more detailed metrics

[mthenw]

This PR adds more detailed metrics, and add support for space label for some of them.

Added metrics (counters):

Functions/subscriptions all metrics are labeled with space label

• gateway_functions_registered_total
• gateway_functions_deleted_total
• gateway_config_function_get_requests_total
• gateway_config_function_register_requests_total
• gateway_config_function_delete_requests_total
• gateway_config_function_update_requests_total
• gateway_config_function_list_requests_total
• gateway_subscriptions_created_total
• gateway_subscriptions_deleted_total
• gateway_config_subscription_get_requests_total
• gateway_config_subscription_create_requests_total
• gateway_config_subscription_delete_requests_total
• gateway_config_subscription_list_requests_total

Events:

• gateway_events_system_function_invoking_received_total
• gateway_events_system_function_invoked_received_total
• gateway_events_system_function_invocation_failed_received_total
• gateway_events_invoke_received_total
• gateway_events_invoke_processed_total
• gateway_events_http_received_total
• gateway_events_http_processed_total

All Events metrics exept gateway_events_http_received_total include space label. It's not possible to determine space for HTTP event then it's received.

Regarding events we already have (all without space label) gateway_events_async_* metrics which were renamed to

• gateway_events_custom_received_total
• gateway_events_custom_processed_total
• gateway_events_custom_dropped_total