PMM-12997 Fix pmm-agent --json output. (#2897) #11443

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, percona/pmm-server:2.0.0)	5m 11s
Tests (mysql:5.7, mongo:4.4, postgres:11, percona/pmm-server:2.0.1)	5m 37s
Tests (mysql:8.0, mongo:4.4, postgres:12, percona/pmm-server:2.1.0)	5m 48s
Tests (mysql:8.0, mongo:4.4, postgres:13, percona/pmm-server:2.2.0)	5m 23s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	5m 39s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	6m 20s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	6m 1s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:dev-latest)	5m 16s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:dev-...	5m 47s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:dev-latest)	5m 23s
	56m 25s

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