remap-lang: allow querying into result of functions

[JeanMertz]

We currently support querying the root object using paths (e.g. .foo.bar) and also variables ($foo.bar since #5094).

We've had at least one report thus far requesting to query the result of functions:

.url = "https://vector.dev"
.scheme = parse_url(.url).scheme // parse_url returns a `Value::Map`
.scheme // "https"

I'm not necessarily advocating we support this (it might be a slippery slope from here to supporting methods, although I don't think that's necessarily the case), but it's worth tracking this, given that more people might be looking for a similar feature.

[JeanMertz]

I forgot to cc @binarylogic and @FungusHumungus for input. I'm also curious what @bruceg and @jszwedko think about this.

[jszwedko]

👍 this seems fairly natural to me.

[binarylogic]

I'm happy with this.

[binarylogic]

@JeanMertz this is not required for 0.12, but if you have time this would be nice to have.

[nnz1024]

We've had at least one report thus far requesting to query the result of functions

Please add me to the list 😃

Now I'm experimenting with Remap and Nginx logs parsing, and sometimes it requires strange constructions like

  upstream_addr_array_last = slice(split(.tmp.upstream_addr, ", "), -1)
  .upstream_addr = upstream_addr_array_last[0]

Here I need to use slice() because []-syntax does not allow to index elements from the end of the array, and also need to use intermediate variable because it is impossible to use slice()[0], which is slightly disappointing.