Logging resource labels use snake_case rather than camelCase

[ofrobots]

Originally reported by @jwgoh in #2484

I tried specifying the monitored resource manually. The keys for labels seems to only take snake_case i.e. project_id and NOT camelCase projectId.

// Works
resource: {
  type: 'gce_instance',
  labels: {
    project_id: projectId,
    instance_id: instanceId,
    zone: zone,
  },
},

// Does not work. Nothing gets logged
resource: {
  type: 'gce_instance',
  labels: {
    projectId: projectId,
    instanceId: instanceId,
    zone: zone,
  },
},

I referred to the documentations here and here when specifying the key-value.