Prefix metric namespaces with tempo instead of cortex

[mapno]

What this PR does:

Changes metrics prefixed with cortex_ to tempo_.

Affected metrics

    "cortex_cache_background_queue_length",
    "cortex_cache_dropped_background_writes_total",
    "cortex_distributor_ingester_clients",
    "cortex_dns_failures_total",
    "cortex_dns_lookups_total",
    "cortex_experimental_features_in_use_total",
    "cortex_kv_request_duration_seconds_bucket",
    "cortex_kv_request_duration_seconds_count",
    "cortex_kv_request_duration_seconds_sum",
    "cortex_member_consul_heartbeats_total",
    "cortex_memcache_client_servers",
    "cortex_memcache_client_set_skip_total",
    "cortex_memcache_request_duration_seconds_bucket",
    "cortex_memcache_request_duration_seconds_count",
    "cortex_memcache_request_duration_seconds_sum",
    "cortex_query_frontend_connected_clients",
    "cortex_query_frontend_discarded_requests_total",
    "cortex_query_frontend_queue_duration_seconds_bucket",
    "cortex_query_frontend_queue_duration_seconds_count",
    "cortex_query_frontend_queue_duration_seconds_sum",
    "cortex_query_frontend_queue_length",
    "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_shutdown_duration_seconds_bucket",
    "cortex_shutdown_duration_seconds_count",
    "cortex_shutdown_duration_seconds_sum"

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[zalegrala]

This fills my heart with joy.

[vainikkaj]

I recently tried to review existing metrics and I totally missed most of these because of their naming 😂