non-zero length test: use `len(s) != 0`

Process completed with exit code 1.

string `--config-file=` has 6 occurrences, make it a constant (goconst)

string `/agent_id/` has 4 occurrences, make it a constant (goconst)

string `.TextFiles.` has 5 occurrences, make it a constant (goconst)

string `cannot use push_metrics_enabled with pmm_agent version=%q,` has 3 occurrences, make it a constant (goconst)

string ` it doesn't support it, minimum supported version=%q` has 3 occurrences, make it a constant (goconst)

string `.ini` has 3 occurrences, make it a constant (goconst)

string `INSERT INTO services (service_id, service_type, service_name, node_id, environment, cluster, replication_set, address, port, socket, external_group, created_at, updated_at) ` has 5 occurrences, make it a constant (goconst)

string `INSERT INTO agents (agent_id, agent_type, runs_on_node_id, pmm_agent_id, node_id, service_id, disabled, status, created_at, updated_at, tls, tls_skip_verify, max_query_length, query_examples_disabled, comments_parsing_disabled, max_query_log_size, table_count_tablestats_group_limit, rds_basic_metrics_disabled, rds_enhanced_metrics_disabled, push_metrics, expose_exporter) ` has 4 occurrences, make it a constant (goconst)

string `INSERT INTO nodes (node_id, node_type, node_name, distro, node_model, az, address, created_at, updated_at) ` has 5 occurrences, make it a constant (goconst)