[kafka_consumer] consumer_groups_regex is applied after fetching offsets for all groups, causing a full admin fan-out

[prabhusiva619]

Title: [kafka_consumer] consumer_groups_regex is applied after fetching offsets for all groups, causing a full admin fan-out

Type: Bug

Summary:
kafka_consumer enumerates every consumer group in the cluster, issues one AdminClient.list_consumer_group_offsets per group, and applies consumer_groups_regex only afterward. As a result a narrow regex (matching a handful of groups) still fans out one admin request per group across the whole cluster. On a Strimzi cluster with ~9,100 consumer groups, a two-group regex still enqueues ~9,100 offset requests.

Impact:

1. The regex does not scope the work, so it is not usable as a way to limit load on large clusters.
2. The mass simultaneous admin-request fan-out is the condition that triggers the librdkafka use-after-free crash on the shared admin coordinator path (rd_kafka_admin_coord_request), see librdkafka Don't assume nested tags exist #4605 / fix PR Remove MIB requirement in profiles #5397. The whole Agent process aborts (exit 139), which on a cluster check runner also takes down co-located checks (kubernetes_state_core, kube_apiserver_metrics, custom_consul).

Expected behaviour:
Filter the group list by consumer_groups_regex before issuing per-group offset requests, so only matching groups incur admin requests.

Notes:
This is fixable on our side without waiting for the upstream librdkafka fix, and it is the change that would let customers use a regex safely on large clusters.

Environment:
kafka_consumer 6.13.0, Agent 7.76.3, confluent-kafka-python / librdkafka 2.11.1, Kafka 3.9.0 (Strimzi), ~9,100 consumer groups.

References:
ZD 2980971, AGENT-16802. Upstream: librdkafka #4605, PR #5397. Adjacent fan-out issues: integrations-core #18983, #19564, #858.