Change default of metrics.namespace

[MichelHollands]

What this PR does / why we need it:

Change the default for a subset of metrics. After this all Loki metrics should start with "loki_". This changes the following metrics:

cortex_distributor_ingester_clients
cortex_dns_failures_total
cortex_dns_lookups_total
cortex_dns_provider_results
cortex_frontend_query_range_duration_seconds_bucket
cortex_frontend_query_range_duration_seconds_count
cortex_frontend_query_range_duration_seconds_sum
cortex_ingester_flush_queue_length
cortex_kv_request_duration_seconds_bucket
cortex_kv_request_duration_seconds_count
cortex_kv_request_duration_seconds_sum
cortex_member_consul_heartbeats_total
cortex_prometheus_last_evaluation_samples
cortex_prometheus_notifications_alertmanagers_discovered
cortex_prometheus_notifications_dropped_total
cortex_prometheus_notifications_errors_total
cortex_prometheus_notifications_latency_seconds
cortex_prometheus_notifications_latency_seconds_count
cortex_prometheus_notifications_latency_seconds_sum
cortex_prometheus_notifications_queue_capacity
cortex_prometheus_notifications_queue_length
cortex_prometheus_notifications_sent_total
cortex_prometheus_rule_evaluation_duration_seconds
cortex_prometheus_rule_evaluation_duration_seconds_count
cortex_prometheus_rule_evaluation_duration_seconds_sum
cortex_prometheus_rule_evaluation_failures_total
cortex_prometheus_rule_evaluations_total
cortex_prometheus_rule_group_duration_seconds
cortex_prometheus_rule_group_duration_seconds_count
cortex_prometheus_rule_group_duration_seconds_sum
cortex_prometheus_rule_group_interval_seconds
cortex_prometheus_rule_group_iterations_missed_total
cortex_prometheus_rule_group_iterations_total
cortex_prometheus_rule_group_last_duration_seconds
cortex_prometheus_rule_group_last_evaluation_timestamp_seconds
cortex_prometheus_rule_group_rules
cortex_query_frontend_connected_schedulers
cortex_query_frontend_queries_in_progress
cortex_query_frontend_retries_bucket
cortex_query_frontend_retries_count
cortex_query_frontend_retries_sum
cortex_query_scheduler_connected_frontend_clients
cortex_query_scheduler_connected_querier_clients
cortex_query_scheduler_inflight_requests
cortex_query_scheduler_inflight_requests_count
cortex_query_scheduler_inflight_requests_sum
cortex_query_scheduler_queue_duration_seconds_bucket
cortex_query_scheduler_queue_duration_seconds_count
cortex_query_scheduler_queue_duration_seconds_sum
cortex_query_scheduler_queue_length
cortex_query_scheduler_running
cortex_quota_cgroup_cpu_max
cortex_quota_cgroup_cpu_period
cortex_quota_cpu_count
cortex_quota_gomaxprocs
cortex_ring_member_heartbeats_total
cortex_ring_member_tokens_owned
cortex_ring_member_tokens_to_own
cortex_ring_members
cortex_ring_oldest_member_timestamp
cortex_ring_tokens_total
cortex_ruler_client_request_duration_seconds_bucket
cortex_ruler_client_request_duration_seconds_count
cortex_ruler_client_request_duration_seconds_sum
cortex_ruler_clients
cortex_ruler_config_last_reload_successful
cortex_ruler_config_last_reload_successful_seconds
cortex_ruler_config_updates_total
cortex_ruler_managers_total
cortex_ruler_ring_check_errors_total
cortex_ruler_sync_rules_total

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR