Proposal: add Flow debug.variables component

[rfratto]

(Originally discussed in #2766)

I propose a new component called debug.variables, which defines a set of variables that can be changed by invoking an API endpoint:

logging {
  level = debug.variables.default.loglevel 
}

debug.variables "default" {
  variable "loglevel" {
    default = "info"
  }
}

Then, one could change the values of variables using the API:

curl -XPOST -d '"debug"' https://localhost:12345/components/debug.variables.default/loglevel

I think a component like this provides some basic level of remote configuration that would be helpful specifically for debugging without having to update the config file. This would also integrate well with conditionals (#2638) to allow dynamically switching between where data get sent:

debug.variables "default" {
  variable "uselocalprom" {
    default = false 
  }
}

prometheus.remote_write "default" {
  client {
    // Syntax for conditionals not final; see grafana/alloy#157 for more details.
    url = (
      if debug.variables.default.uselocalprom 
      then env("LOCAL_PROMETHEUS") 
      else env("CLOUD_PROMETHEUS")
    )
  }
}

Details

The variable block is a labeled block which defines an individual variable whose value can change. The label of the block defines the name of the variable. The default attribute provides the original value for the variable. The default attribute is ignored if a user has provided an explicit value via the API.

Variable values must be a type which can be represented in JSON:

• Boolean
• Number
• String
• Object
• Array

Arrays can only have elements matching any of the above types, and Objects can only have values matching any of the above types.

For the initial implementation, it will not be possible to use River expressions from the API, so no capsule values can be used.

UI integration

The UI could show a special view for debug.variables which would allow changing the values for variables within the UI itself (and showing the current values).

[mattdurham]

I like this, the only part I am a bit hesitant about is the UI integration.

[rfratto]

As discussed during the proposal review in the community call, we think that this encourages bad behavior, where debug hooks are left in the config file permanently. We think that changing the config file when you need to test out another value is a healthier approach, so we decided to reject this for now.