Prometheus plugin - series with +Inf , -Inf, NaN values fail for json output as its not a valid json for value

[vishiy]

Relevant telegraf.conf:

###############################################################################
#                            OUTPUT PLUGINS                                   #
###############################################################################

# Generic socket writer capable of handling multiple socket types.
[[outputs.socket_writer]]
  ## URL to connect to
  address = "tcp://0.0.0.0:25226"
  

  ## Optional TLS Config
  # tls_ca = "/etc/telegraf/ca.pem"
  # tls_cert = "/etc/telegraf/cert.pem"
  # tls_key = "/etc/telegraf/key.pem"
  ## Use TLS but skip chain & host verification
  # insecure_skip_verify = false

  ## Period between keep alive probes.
  ## Only applies to TCP sockets.
  ## 0 disables keep alive probes.
  ## Defaults to the OS configuration.
  # keep_alive_period = "5m"

  ## Data format to generate.
  ## Each data format has its own unique set of configuration options, read
  ## more about them here:
  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
  data_format = "json"

[[inputs.prometheus]]
  interval = "1m"
  
  ## Scrape Kubernetes pods for the following prometheus annotations:
  ## - prometheus.io/scrape: Enable scraping for this pod
  ## - prometheus.io/scheme: If the metrics endpoint is secured then you will need to
  ##     set this to `https` & most likely set the tls config.
  ## - prometheus.io/path: If the metrics path is not /metrics, define it with this annotation.
  ## - prometheus.io/port: If port is not 9102 use this annotation
  monitor_kubernetes_pods = true
  metric_version = 2
  ## Kubernetes config file to create client from.
  # kube_config = "/path/to/kubernetes.config"
  ## Use bearer token for authorization. ('bearer_token' takes priority)
  bearer_token = "/var/run/secrets/kubernetes.io/serviceaccount/token"
  ## OR
  # bearer_token_string = "abc_123"
  ## Specify timeout duration for slower prometheus clients (default is 3s)
  response_timeout = "15s"

  ## Optional TLS Config
  tls_ca = "/var/run/secrets/kubernetes.io/serviceaccount/ca.crt"
  #tls_cert = /path/to/certfile
  # tls_key = /path/to/keyfile
  ## Use TLS but skip chain & host verification
  insecure_skip_verify = true

System info:

Docker

Steps to reproduce:

1. Here is a gauge metric with -ve infinity as value (-Inf) - This is a correct value as per prometheus line format (+Inf , -Inf & NaN are valid values)

TYPE flink_taskmanager_job_task_operator_KafkaProducer_record_size_max gauge

flink_taskmanager_job_task_operator_KafkaProducer_record_size_max{job_id="40c668ac8435480261a4c2afb8f88deb",task_id="c8e26e1de46a579b823e994ab7ebd862",task_attempt_id="f099f1bb3403a8762dfc12af3a580c29",host="flink_tm_0",operator_id="fd06000fa5e590dc6d2474670a0b4c2c",operator_name="Sink:_Unnamed",task_name="KeyedProcess_____Sink:_Unnamed__Sink:Unnamed",task_attempt_num="0",job_name="pulse_TictracService",tm_id="7f41cff8bb3199807ca9b9bb164eed23",subtask_index="2",} -Inf

2. Stitch the prometheus input with json output format, and you will see the below error -
   2020-04-21T01:01:05Z E! [agent] Error writing to output [socket_writer]: json: unsupported value: -Inf

Expected behavior:

No error when serializing as JSON
Looks to me we are parsing values using float64() rather than parseFloat(), which is the root cause.