[azure_monitor] Cannot use empty string as the nampspace prefix

[ethanshea-ms]

Relevant telegraf.conf:

[[outputs.azure_monitor]]
  namespace_prefix = ""

# For debugging
[[outputs.file]]
  data_format = "influx"
  files = ["stdout"]

System info:

Telegraf 1.15.3

Steps to reproduce:

1. Run telegraf with the above configuration

Expected behavior:

Metrics are emitted to azure monitor in the AZM namespace which matches the field name.

Actual behavior:

Metrics are emitted to azure monitor in the AZM namespace telegraf/{fieldName}.

Additional info:

This seems to be the suspect part of the code. Can it be compared to nil instead of the empty string?

As a side note, the regex processor may be a better solution to add namespace prefixes.

ATTN: @ danielnelson

[ssoroka]

This seems to be the suspect part of the code. Can it be compared to nil instead of the empty string?
strings can't be nil in go, unless they're defined as *string, so I can see how this would be a problem.

I would have preferred not having a default set. Options for a fix are:
a) have a new "ignore_prefix = true" option. Kinda crappy.
b) change the default to blank (breaking change)
c) make users set it to " " (space) to mean blank (unintuitive; I kind of hate this option)
d) try changing the type to *string and do a nil check (will have to confirm this works correctly with the TOML parser)
e) move the default setting to the Init() error or plugin creation step. This might be the cleanest as I think it will just work properly with no side effects (assuming the TOML parser works properly)

that's all I can think of.

For a fix I'd probably try e, then d.

[ssoroka]

Update: I've crossed out the options that don't make sense. I think the only reasonable option is e, which I'm like 98% sure will just work properly as a one-line fix, and then we can remove the if conditional check.