As we are not constrained by time that much in the outlet, we can
simplify the fetching of metadata by doing it synchronously. We still
keep the breaker design to avoid continously polling a source that is
not responsive, so we still can loose some data if we are not able to
poll metadata. We also keep the background cache refresh. We also
introduce a grace time of 1 minute to avoid loosing data during start.
For the static provider, we wait for the remote data sources to be
ready. For the gNMI provider, there are target windows of availability
during which the cached data can be polled. The SNMP provider is loosing
its ability to coalesce requests.
For example:
```
17:35 ❱ curl -s 127.0.0.1:8080/api/v0/outlet/metrics | promtool check metrics
akvorado_outlet_core_classifier_exporter_cache_size_items counter metrics should have "_total" suffix
akvorado_outlet_core_classifier_interface_cache_size_items counter metrics should have "_total" suffix
akvorado_outlet_flow_decoder_netflow_flowset_records_sum counter metrics should have "_total" suffix
akvorado_outlet_flow_decoder_netflow_flowset_records_sum non-histogram and non-summary metrics should not have "_sum" suffix
akvorado_outlet_flow_decoder_netflow_flowset_sum counter metrics should have "_total" suffix
akvorado_outlet_flow_decoder_netflow_flowset_sum non-histogram and non-summary metrics should not have "_sum" suffix
akvorado_outlet_kafka_buffered_fetch_records_total non-counter metrics should not have "_total" suffix
akvorado_outlet_kafka_buffered_produce_records_total non-counter metrics should not have "_total" suffix
akvorado_outlet_metadata_cache_refreshs counter metrics should have "_total" suffix
akvorado_outlet_routing_provider_bmp_peers_total non-counter metrics should not have "_total" suffix
akvorado_outlet_routing_provider_bmp_routes_total non-counter metrics should not have "_total" suffix
```
Also ensure metrics using errors as label don't have a too great
cardinality by using constants for error messages used.
