Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

etc/telegraf.conf

@@ -30,12 +30,15 @@
   ## ie, if interval="10s" then always collect on :00, :10, :20, etc.
   round_interval = true
 
-  ## Telegraf will send metrics to outputs in batches of at
-  ## most metric_batch_size metrics.
+  ## Telegraf will send metrics to outputs in batches of at most
+  ## metric_batch_size metrics.
+  ## This controls the size of writes that Telegraf sends to output plugins.
   metric_batch_size = 1000
+
   ## For failed writes, telegraf will cache metric_buffer_limit metrics for each
   ## output, and will flush this buffer on a successful write. Oldest metrics
   ## are dropped first when this buffer fills.
+  ## This buffer only fills when writes fail to output plugin(s).
   metric_buffer_limit = 10000
 
   ## Collection jitter is used to jitter the collection by a random amount.
@@ -57,10 +60,15 @@
   ## Precision will NOT be used for service inputs, such as logparser and statsd.
   ## Valid values are "ns", "us" (or "µs"), "ms", "s".
   precision = ""
-  ## Run telegraf in debug mode
+
+  ## Logging configuration:
+  ## Run telegraf with debug log messages.
   debug = false
-  ## Run telegraf in quiet mode
+  ## Run telegraf in quiet mode (error log messages only).
   quiet = false
+  ## Specify the log file name. The empty string means to log to stdout.
+  logfile = ""
+
   ## Override default hostname, if empty use os.Hostname()
   hostname = ""
   ## If set to true, do no set the "host" tag in the telegraf agent.
@@ -1064,8 +1072,6 @@
 #   #   "tasks",
 #   #   "messages",
 #   # ]
-#   ## Include mesos tasks statistics, default is false
-#   # slave_tasks = true
 
 
 # # Read metrics from one or many MongoDB servers
@@ -1442,25 +1448,29 @@
 # # Retrieves SNMP values from remote agents
 # [[inputs.snmp]]
 #   agents = [ "127.0.0.1:161" ]
+#   ## Timeout for each SNMP query.
 #   timeout = "5s"
+#   ## Number of retries to attempt within timeout.
+#   retries = 3
+#   ## SNMP version, values can be 1, 2, or 3
 #   version = 2
 #
-#   # SNMPv1 & SNMPv2 parameters
+#   ## SNMP community string.
 #   community = "public"
 #
-#   # SNMPv2 & SNMPv3 parameters
-#   max_repetitions = 50
+#   ## The GETBULK max-repetitions parameter
+#   max_repetitions = 10
 #
-#   # SNMPv3 parameters
+#   ## SNMPv3 auth parameters
 #   #sec_name = "myuser"
-#   #auth_protocol = "md5"         # Values: "MD5", "SHA", ""
-#   #auth_password = "password123"
-#   #sec_level = "authNoPriv"      # Values: "noAuthNoPriv", "authNoPriv", "authPriv"
+#   #auth_protocol = "md5"      # Values: "MD5", "SHA", ""
+#   #auth_password = "pass"
+#   #sec_level = "authNoPriv"   # Values: "noAuthNoPriv", "authNoPriv", "authPriv"
 #   #context_name = ""
-#   #priv_protocol = ""            # Values: "DES", "AES", ""
+#   #priv_protocol = ""         # Values: "DES", "AES", ""
 #   #priv_password = ""
 #
-#   # measurement name
+#   ## measurement name
 #   name = "system"
 #   [[inputs.snmp.field]]
 #     name = "hostname"
@@ -1475,7 +1485,7 @@
 #     oid = "HOST-RESOURCES-MIB::hrMemorySize"
 #
 #   [[inputs.snmp.table]]
-#     # measurement name
+#     ## measurement name
 #     name = "remote_servers"
 #     inherit_tags = [ "hostname" ]
 #     [[inputs.snmp.table.field]]
@@ -1490,7 +1500,7 @@
 #       oid = ".1.0.0.0.1.2"
 #
 #   [[inputs.snmp.table]]
-#     # auto populate table's fields using the MIB
+#     ## auto populate table's fields using the MIB
 #     oid = "HOST-RESOURCES-MIB::hrNetworkTable"