lmcp-api#236. Read-only.

• update_failures: failed update attempts (error/refused, not "the previous update was never applied") of the last days, by country, ASN and error, counted in machines and attempts. by_country / by_asn are the same failures rolled up.
• machines_without_api: machines whose every event of the last recent_hours arrived via the backchannel or delayed from the local queue (total is the count before limit).
• coverage: how many events of the window carry a country / an ASN.
• probe: the clients' connectivity matrix by country, ASN, target, stage and code, in machines.

Test machines (metrics.test_exclusion) are left out unless include_test.