add perf feature-flag; ensure compatability with LaunchDarkly (LDv5) users

[colesnodgrass]

What

• part of Performance - background json schema validation #21492
• update feature-flag client capabilities
  • updated ConfigFileClient to support passing in a null config file path
    • if a null config is provided, the requested feature-flag default values will always be returned
    • this was changed to make instantiating the FeatureFlagClient easier
  • enable the Multi contexts to work with LDv5
    • convert the context-key to LDv5 user attributes so feature-flags can be correctly targeted
  • pass LDv6 contexts as LDv5 user attributes
    • as our LaunchDarkly account does not yet have access to contexts, this ensures that our Context attributes are set correctly on the LaunchDarkly user
• add performance.backgroundJsonSchemaValidation feature flag
  • to both the flags.yml and Flags.kt files

Recommended reading order

1. Flags.kt
2. flags.yml
3. Context.kt
4. Client.kt
5. tests

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[jdpgrailsdev]

Out of curiosity, Kotlin doesn't have a more succinct way to represent this type of if/else? Or, are you just avoiding using a ternary or elvis operator for stylistic/readability reasons?

[colesnodgrass]

Kotlin doesn't have a ternary operator, instead having an if/else and when expressions. The elvis operator only works when the left-hand side of the operator is null. As the configPath variables cannot be null (it's a String not a String?) I would have to convert it to a null in order to use a elvis operator.

Here I think it's a stylistic/readability choice.

Which one is easier to read?
if/else:

val path: Path? = if (configPath.isNotBlank()) {
  Path.of(configPath)
} else {
  null
}

when:

val path: Path? = when {
  configPath.isNotBlank() -> Path.of(configPath)
  else -> null
}

elvis (actually doesn't need an elvis as we want the null value)

val path: Path? = configPath.takeIf { it.isNotBlank() }?.let { Path.of(it) }

Having written these all out now, maybe I'll change this to use the when statement.

[jdpgrailsdev]