[pkg/ottl] Add the When OTTL converter

[edmocosta]

Description

Added the When OTTL converter for selecting a value based on a lambda condition.
This function takes no arguments, and is meant to be used as a helper-function to supply ternary operator-like support to other lambda function or value operation that requires evaluating a condition, so it's more flexible and does not require multiple statements for reaching the same results, a few examples:

set(log.attributes["env"], When(
  () => resource.attributes["deployment.environment"] == "prod",
  "production",
  "non-production"
)) where resource.attributes["service.name"] == "foo"

set(log.attributes["exported"], When(
  () => IsMap(log.body),
  Filter(log.attributes, (k, _) => HasPrefix(k, "http.")),
  log.body
)) where resource.attributes["service.name"] == "foo"

set(log.attributes["tier"], When(
  () => Any(log.attributes["tags"], (_, v) => v == "admin"),
  "privileged",
  "standard"
))

Testing

Unit and E2E

Documentation

Updated README.md

Authorship

• I, a human, wrote this pull request description myself.

[edmocosta]

Please do not merge it yet, we need to adapt it to #49421 (breaking change) first.