Issues upgrading from 0.16.1 to 0.17.3

[mhratson]

Issues encountered

They weren't mentioned in upgrade guide so i'm documenting it here for there record.

1 Undocumented Rename

Before we had rtt_threshold_ratio in our sinks.datadog-metrics sink. After we're getting error:

   Running tests
   Oct 25 23:58:28.508 ERROR vector::unit_test: Failed to execute tests:
   unknown field `rtt_threshold_ratio`, expected one of `decrease_ratio`, `ewma_alpha`, `rtt_deviation_scale` for key `sinks.datadog-metrics` at line 45 column 1.

Solution: to use rtt_deviation_scale instead.

2 nil is no longer accepted as VRL

 error[E701]: call to undefined variable
     ┌─ :10:14
     │
  10 │    if err == nil {
     │              ^^^
     │              │
     │              undefined variable
     │              did you mean "err"?
     │
     = see language documentation at https://vrl.dev

Solution: use null instead.

[bruceg]

1. That is correct, and we missed that in the upgrade notes.
2. This one is a bit of an accident. nil was never valid VRL. However it was previously parsed as an undeclared variable, and all such variables contained the value null. In 0.17, VRL now checks for undeclared variables and produces an error.