PMM-12997 Fix pmm-agent --json output. (#2897) #11473

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, percona/pmm-server:2.0.0)	6m 43s
Tests (mysql:5.7, mongo:4.4, postgres:11, percona/pmm-server:2.0.1)	6m 43s
Tests (mysql:8.0, mongo:4.4, postgres:12, percona/pmm-server:2.1.0)	6m 55s
Tests (mysql:8.0, mongo:4.4, postgres:13, percona/pmm-server:2.2.0)	6m 39s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	6m 49s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	7m 19s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	7m 17s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:dev-latest)	6m 46s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:dev-...	6m 44s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:dev-latest)	7m 2s
	1h 8m 57s

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