vSphere Plugin: Samples missing from non-realtime metrics.

[prydin]

Relevant telegraf.conf:

# Read metrics from VMware vCenter
[[inputs.vsphere]]
  interval = "20s"
  ## List of vCenter URLs to be monitored. These three lines must be uncommented
  ## and edited for the plugin to work.
  vcenters = [ "https://example.com:8989/sdk" ]
  username = "user@corp.local"
  password = "secret"

  ## VMs
  ## Typical VM metrics (if omitted or empty, all metrics are collected)
  vm_metric_include = ["*"]
  
  # vm_instances = true ## true by default

  ## Hosts
  ## Typical host metrics (if omitted or empty, all metrics are collected)
  host_metric_include = [ "*" ]
  
# host_metric_exclude = [] ## Nothing excluded by default
  # host_instances = true ## true by default

  ## Clusters
  cluster_metric_include = [ "*" ] ## if omitted or empty, all metrics are collected
  # cluster_metric_exclude = [] ## Nothing excluded by default
  # cluster_instances = true ## true by default

  ## Datastores
  datastore_metric_include = [ "*" ] ## if omitted or empty, all metrics are collected
  # datastore_metric_exclude = [] ## Nothing excluded by default
  # datastore_instances = false ## false by default for Datastores only

  ## Datacenters
  datacenter_metric_include = [ "*" ] ## if omitted or empty, all metrics are collected
  # datacenter_instances = false ## false by default for Datastores only

  ## Plugin Settings
  ## separator character to use for measurement and field names (default: "_")
  # separator = "_"

  ## number of objects to retreive per query for realtime resources (vms and hosts)
  ## set to 64 for vCenter 5.5 and 6.0 (default: 256)
  # max_query_objects = 256

  ## number of metrics to retreive per query for non-realtime resources (clusters and datastores)
  ## set to 64 for vCenter 5.5 and 6.0 (default: 256)
  # max_query_metrics = 256

  ## number of go routines to use for collection and discovery of objects and metrics
  collect_concurrency = 3
  discover_concurrency = 3

  ## whether or not to force discovery of new objects on initial gather call before collecting metrics
  ## when true for large environments this may cause errors for time elapsed while collecting metrics
  ## when false (default) the first collection cycle may result in no or limited metrics while objects are discovered
  # force_discover_on_init = false

  ## the interval before (re)discovering objects subject to metrics collection (default: 300s)
  # object_discovery_interval = "300s"

  ## timeout applies to any of the api request made to vcenter
  # timeout = "20s"

  ## Optional SSL Config
  # ssl_ca = "/path/to/cafile"
  # ssl_cert = "/path/to/certfile"
  # ssl_key = "/path/to/keyfile"
  ## Use SSL but skip chain & host verification
  insecure_skip_verify = true

System info:

Ubuntu 16.04 AWS "Small" configuration.
Telegraf 1.18

Steps to reproduce:

Collect some non-realtime metrics, e.g. datastores or clusters.

Expected behavior:

You should see a data point once every 5 minutes.

Actual behavior:

Some data points are missing.

Additional info:

This bug is due to some strangeness in the way vCenter handles the latest sample in non-realtime metrics. Details: https://kb.vmware.com/s/article/2009550