Don't crash when a probe receives undefined

[ajpalkovic]

I noticed my service is crashing from time to time because a probe gets an "undefined" value.
It looks like the prob is configured to call ETS.info(:size), which the erlang docs say can return :undefined. Since most services probably have this problem, figured it's best to just drop :undefined like we would nil in the probe.

[ajpalkovic]

Polite ping :)

[ihumanable]

I'm a bit confused here, probe_modules should implement the Instruments.Probe behaviour which specifies that probe_get_value should return {:ok, probe_value} where probe_value is defined as number | keyword.

It sounds like the probe module is returning {:ok, :undefined} which violates the behaviour contract, why not just fix the probe?

[ajpalkovic]

I don't completely remember, but I'm pretty sure the problem was:
I have a probe definition in discord_lfg.ex:

    Probe.define!(
      "discord_lfg.guilds.count",
      :gauge,
      mfa: {DiscordLfg, :count_guilds, []},
      report_interval: 10_000
    )

count_guilds is defined as:

  def count_guilds() do
    @registry_module.count(GuildLfg)
  end

@registry_module is a GenRegistry

GenRegistry calls into ETS, and ETS returns :undefined if the "thing" has no entries yet

So, I figured other places probably have the same problem so rather than fix all of them, might as well just change the probe to accept :undefined and treat it the same as nil? I don't really feel that strongly about it though

[ihumanable]

Since instruments is an open-source project that has many consumers, I think obeying the contract in this case instead of making an exception for GenRegistry / ETS is the correct course of action here.