fix mongo exporter auth in secondary mongod

[hzhao-wish]

The current mongoexporter cannot log in the secondary mongod as it does not have permission.

The default mode is that reads and writes will always be made to the primary mongod. As we force mongoexporter to directly connect to the local mongod, when the local mongod is secondary, it does not have permission to login.

In order to fix this, we need to set mode before log in. Mgo requries that the consistency requirements set to be Monotonic or Eventual for direct connection. Set session mode to be eventual before login can fix the problem.

Test in a tools secondary mongod (4cfe1d6c.sjc4.w.wish.com):

hzhao@4cfe1d6c:~$ ./mongodb_exporter --mongodb.maxtimems=1000ms --mongodb.collect.database=True --mongodb.collect.collection=True --mongodb.uri=mongodb://localhost:27017 --mongodb.tls-disable-hostname-validation=True --mongodb.mechanism=MONGODB-X509 --mongodb.collect.top=True --mongodb.tls-auth=True --mongodb.collect.oplog_tail=True --mongodb.tls-cert=/etc/mongo_secrets/mongoexporter_cert.conf --mongodb.socket-timeout=1000ms --mongodb.tls-ca=/etc/mongo_secrets/mongoexporter_ca_chain.conf --logtostderr
Listening on :9001 (scheme=HTTP, secured=no, clientValidation=no)
I1022 11:50:57.050271   37161 mongodb_collector.go:104] Collecting CurrentOp Info
I1022 11:50:57.050780   37161 mongodb_collector.go:163] exporting CurrentOp Metrics
I1022 11:50:57.050798   37161 mongodb_collector.go:106] Collecting Server Status
I1022 11:50:57.052066   37161 mongodb_collector.go:172] exporting ServerStatus Metrics
I1022 11:50:57.052273   37161 mongodb_collector.go:109] Collecting ReplSet Status
I1022 11:50:57.052556   37161 mongodb_collector.go:193] exporting ReplSetStatus Metrics
I1022 11:50:57.052859   37161 mongodb_collector.go:204] exporting ReplSetConf Metrics
...