splunkmetric serializer doesn't properly support bool values

[ronnocol]

Relevant telegraf.conf:

[[outputs.file]]
files = ["stdout"]
data_format = "splunkmetric"
[[inputs.docker]]

System info:

Telegraf unknown (git: master f175848)
Linux or OSX
Splunk 7.1+ (with metrics support)

Steps to reproduce:

1. Run telegraf with an input that produces a bool (e.g. inputs.docker)
2. Watch Splunk be sad with a non-numeric metric value

Expected behavior:

The metric is either converted into a valid value (1 for true, 0 for false) or dropped like string values are.

Actual behavior:

Serializer tries to send the metric to Splunk causing an ingest error.

Additional info:

Reported to me on the Splunk Slack channels by guilhem.marchand