Environment variables aren't interpolated in plugin ID field

When loading the node pipeline stats via `/_node/stats/pipeline`, environment variables used within the ID field of the plugin don't seem to be interpolated. In my case, I use a env var `${LOGSTASH_TENANT}` to indicate the tenant name, when I have multiple internal tenants (in a Docker use-case).

Version: 5.2.2
Operating System: Darwin

**Sample Logstash Config: (indexing.conf)**
```
input {
kafka {
id => "${LOGSTASH_TENANT}_indexing_input_kafka1"
group_id => "cg-${LOGSTASH_TENANT}-indexing"
decorate_events => true
consumer_threads => "${LOGSTASH_INDEXER_KAFKA_CONSUMER_THREADS}"
bootstrap_servers => "${LOGSTASH_KAFKA_BROKERS_LIST}"
topics => [ "logstash-${LOGSTASH_TENANT}-indexing" ]
codec => "json"
}
}

filter {
# ************* The following block deals with emtpy messages *************
grok {
id => "${LOGSTASH_TENANT}_indexing_filter_grok1"
match => [
"message", "^\s*$"
]
add_tag => ["empty_message"]
tag_on_failure => ["not_empty"]
}

if "empty_message" in [tags] {
drop {}
}

if "not_empty" in [tags] {
mutate {
id => "${LOGSTASH_TENANT}_indexing_filter_mutate1"
remove_tag => [ "not_empty" ]
}
}

# ************* The following section deals with adding GeoIP data *************
if [clientip] {
mutate {
id => "${LOGSTASH_TENANT}_indexing_filter_mutate2"
add_field => {
"_ip_field" => "%{clientip}"
}
}
}
if [client_ip] {
mutate {
id => "${LOGSTASH_TENANT}_indexing_filter_mutate3"
add_field => {
"_ip_field" => "%{client_ip}"
}
}
}

if [_ip_field] {
geoip {
id => "${LOGSTASH_TENANT}_indexing_filter_geoip1"
source => "%{_ip_field}"
target => "geoip"
fields => ["city_name", "continent_code", "country_code2", "dma_code", "ip", "latitude", "longitude", "region_name", "region_code", "timezone", "location"]
add_field => [ "[geoip][coordinates]", "%{[geoip][longitude]}" ]
add_field => [ "[geoip][coordinates]", "%{[geoip][latitude]}" ]
lru_cache_size => 5000
}
mutate {
id => "${LOGSTASH_TENANT}_indexing_filter_mutate4"
convert => [ "[geoip][coordinates]", "float" ]
}
mutate {
id => "${LOGSTASH_TENANT}_indexing_filter_mutate5"
remove_field => [ "_ip_field" ]
}
}

de_dot {
id => "${LOGSTASH_TENANT}_indexing_filter_dedot1"
add_tag => ["de_dot"]
}

# ************* Finally, generate a UUID for this event, which will be used as value for _id *************
uuid {
id => "${LOGSTASH_TENANT}_indexing_filter_uuid1"
#target => "@uuid"
target => "[@metadata][uuid]"
}
}

output {
stdout { codec => rubydebug }
elasticsearch {
id => "${LOGSTASH_TENANT}_indexing_output_elasticsearch1"
hosts => [ "${LOGSTASH_ES_HOST}" ]
manage_template => "false"
action => "update"
doc_as_upsert => true
document_id => "%{[@metadata][uuid]}"
index => "%{team}-%{environment}-%{type}-%{+YYYY.MM.dd}"
}
}
```

**Sample Logstash Startup Command** (for testing):
```
LOGSTASH_TENANT="team1" \
LOGSTASH_ENVIRONMENT="development" \
LOGSTASH_INDEXER_KAFKA_CONSUMER_THREADS=2 \ LOGSTASH_KAFKA_BROKERS_LIST="localhost:9092" \
LOGSTASH_ES_HOST="https://localhost:9200" \
./bin/logstash -f indexing.conf --log.level=info --log.format=json
```

**Sample Logstash Stats:**
```
{
"host": "dev.local",
"version": "5.2.2",
"http_address": "127.0.0.1:9600",
"id": "cd23651e-09e6-4da9-ace5-3bcc34296809",
"name": "dev.local",
"pipeline": {
"events": {
"duration_in_millis": 0,
"in": 0,
"filtered": 0,
"out": 0
},
"plugins": {
"inputs": [],
"filters": [
{
"id": "${LOGSTASH_TENANT}_indexing_filter_geoip1",
"name": "geoip"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-3",
"name": "drop"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-12",
"name": "mutate"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_mutate1",
"name": "mutate"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_mutate3",
"name": "mutate"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_dedot1",
"name": "de_dot"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-14",
"name": "date"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_mutate5",
"name": "mutate"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-13",
"name": "date"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-11",
"name": "mutate"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_grok1",
"patterns_per_field": {
"message": 1
},
"name": "grok"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_uuid1",
"name": "uuid"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_mutate4",
"name": "mutate"
},
{
"id": "${LOGSTASH_TENANT}_indexing_filter_mutate2",
"name": "mutate"
}
],
"outputs": [
{
"id": "${LOGSTASH_TENANT}_indexing_output_elasticsearch1",
"name": "elasticsearch"
},
{
"id": "2976b184398a75080278c36d732a58060d2bab9c-16",
"name": "stdout"
}
]
},
"reloads": {
"last_error": null,
"successes": 0,
"last_success_timestamp": null,
"last_failure_timestamp": null,
"failures": 0
},
"queue": {
"type": "memory"
}
}
}
```