feat: Add new stdlib function to_json

[ravishankar15]

PR Description

Adds a new stdlib function for json encoding to_json which converts a map to JSON

Which issue(s) this PR fixes

Fixes #152

Notes to the Reviewer

PR Checklist

• CHANGELOG.md updated
• Documentation added
• Tests updated

[wildum]

Thanks for your contribution, the code looks good but it looks like it can handle other types that map. Could you add tests for the different types and document the ones that don't work? (Not sure what happens if you try to encode a Secret or a Capsule for example)

[ravishankar15]

I have updated the tests and documentation. For Secret and Capsule the function converts the public fields of the underlying struct. I updated the doc with the closest understandable text. Should we handle this differently ?

Example configuration the I tried for verifying the behaviour,

discovery.relabel "keep_backend_only" {
  targets = [
    { "__meta_foo" = "foo", "__address__" = "localhost", "instance" = "ones",   "app" = "backend" },
    { "__meta_bar" = "bar", "__address__" = "localhost", "instance" = "two",   "app" = "database" },
    { "__meta_baz" = "baz", "__address__" = "localhost", "instance" = "three", "app" = "frontend" },
  ]

  rule {
    source_labels = ["__address__", "instance"]
    separator     = "/"
    target_label  = "destination"
    action        = "replace"
  }

  rule {
    source_labels = ["app"]
    action        = "keep"
    regex         = "backend"
  }
}

declare "discovery_result" {
  argument "output_value" {
    optional = false
  }

  argument "rules_value" {
    optional = false
  }

  argument "number_value" {
    optional = true
    default = 14
  }

  argument "array_value" {
    optional = true
  }

  argument "string_value" {
    optional = true
  }

  argument "null_value" {
    optional = true
  }
}

discovery_result "backend" {
  output_value = encoding.to_json(discovery.relabel.keep_backend_only.output)
  rules_value = encoding.to_json(discovery.relabel.keep_backend_only.rules)
  number_value = encoding.to_json(23)
  array_value = encoding.to_json([1,2,3])
  string_value = encoding.to_json("Hello World")
  null_value = encoding.to_json(null)
}

[wildum]

Thanks for testing it, I think that the best approach would be to prevent capsule and secret/optional secrets from being converted because the only way to expose a secret should be through the nonsensitive function + I don't see a good usage to get the public fields exported of the capsule for now.

[ravishankar15]

I made the changes to support only map type instead of complicating the support for all other types.

The way i see it,

1. we can add conditions on top of map[string]interface{} for other types int, float, string etc,
2. We can have a recursive logic to convert the applicable types types since a map can have have nested maps and arrays can have map nested.

Let me know your thoughts or if we have any other approaches here ?

[clayton-cornell]

Suggested change

	For example, `config` of a [`prometheus.exporter.blackbox`][]
	For example, configuration of a [`prometheus.exporter.blackbox`][]

Missing a word here... a prometheus.exporter.blackbox string?

[ravishankar15]

I wanted to refer the param name in the component updated accordingly.