bug: Reduce transform has unexpected behavior on arrays

[Hoverbear]

Given the file test.json with content:

{ "operations": ["insert", "select"] }
{ "operations": ["select"] }
{ "operations": ["select"] }
{ "operations": ["select"] }
{ "operations": ["select"] }
{ "operations": ["select"] }
{ "operations": ["select"] }
{ "operations": ["drop"], "transaction_state": "commit" }

And the configuration of:

data_dir = "var"

[sources.database_file]
  type = "file"
  include = ["test.json"]


[transforms.database_log]
  type = "json_parser"
  inputs = ["database_file"]

[transforms.reducer]
  type = "reduce"
  inputs = ["database_log"]
  merge_strategies."operations" = "concat"
  ends_when.type = "check_fields"
  ends_when."transaction_state.eq" = "commit"

[sinks.sink0]
  inputs = ["reducer"]
  type = "console"
  encoding = "json"

On latest main branch checkout, seems to produce unexpected output.

hoverbear@OBSIDIAN:/git/timberio/vector$ ./target/debug/vector --config test.toml 
Jul 15 13:07:36.188  INFO vector: Log level "info" is enabled.
Jul 15 13:07:36.191  INFO vector: Loading configs. path=["test.toml"]
Jul 15 13:07:36.204  INFO vector: Vector is starting. version="0.10.0" git_version="v0.9.0-341-g6358cac" released="Mon, 13 Jul 2020 20:39:08 +0000" arch="x86_64"
Jul 15 13:07:36.205  INFO vector::topology: Running healthchecks.
Jul 15 13:07:36.205  INFO vector::topology: Starting source "database_file"
Jul 15 13:07:36.205  INFO vector::topology::builder: Healthcheck: Passed.
Jul 15 13:07:36.206  INFO vector::topology: Starting transform "database_log"
Jul 15 13:07:36.206  INFO vector::topology: Starting transform "reducer"
Jul 15 13:07:36.206  INFO source{name=database_file type=file}: vector::sources::file: Starting file server. include=["test.json"] exclude=[]
Jul 15 13:07:36.206  INFO vector::topology: Starting sink "sink0"
Jul 15 13:07:36.208  INFO source{name=database_file type=file}:file_server: file_source::file_server: Found file to watch. path="test.json" file_position=0
{"file":"test.json","host":"OBSIDIAN","operations":["insert","select"],"source_type":"file","timestamp":"2020-07-15T20:07:36.223375960Z","timestamp_end":"2020-07-15T20:07:36.224779276Z","transaction_state":"commit"}

Expected output should have the final line as :

{"file":"test.json","host":"OBSIDIAN","operations":["insert","select","select","select","select","select","select","select","drop"],"source_type":"file","timestamp":"2020-07-15T20:07:36.223375960Z","timestamp_end":"2020-07-15T20:07:36.224779276Z","transaction_state":"commit"}

[Hoverbear]

@lukesteensen Do you want to block #2961 on this?

[Hoverbear]

Blocks #3022