Write error metrics

Telegraf
1

I have a Telegraf agent v1.34 deployed with the Influx helm chart v1.8.55. There is a syslog input plugin and several output plugins (pubsub, health and prometheus) as shown below:

[agent]
      buffer_strategy = "memory"
      collection_jitter = "0s"
      debug = true
      flush_interval = "20s"
      flush_jitter = "0s"
      hostname = "$HOSTNAME"
      interval = "10s"
      logfile = ""
      metric_batch_size = 1000
      metric_buffer_limit = 25000
      omit_hostname = false
      precision = ""
      quiet = false
      round_interval = true
    [[processors.enum]]
       [[processors.enum.mapping]]
        dest = "status_code"
        field = "status"
        [processors.enum.mapping.value_mappings]
            critical = 3
            healthy = 1
            problem = 2
    [[outputs.cloud_pubsub]]
      content_encoding = "identity"
      credentials_file = "/config/gcp/credentials.json"
      data_format = "json"
      namedrop = [
        "internal*"
      ]
      project = "xxxx"
      topic = "yyyyy"
    [[outputs.prometheus_client]]
      collectors_exclude = [
        "gocollector"
      ]
      listen = ":9273"
      path = "/metrics"

    [[outputs.health]]
      service_address = "http://:8888"
      namepass = ["internal_write"]
    [[outputs.health.compares]]
        field = "buffer_size"
        lt = 20000.0
    [[outputs.health.contains]]
        field = "buffer_size"
    [[inputs.syslog]]
      server = "udp://:514"
      syslog_standard = "RFC3164"
    [[inputs.internal]]
      collect_memstats = false

In normal conditions I can query Prometheus and retrieve input metrics (internal_gather_metrics_gathered) and output metrics (internal_write_metrics_written). However, I have a situation where the cloud_pubsub plugin errors:

2025-07-03T10:53:30Z E! [agent] Error writing to outputs.cloud_pubsub: rpc error: code = Unauthenticated desc = Request had invalid authentication credentials. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.

When this issue happens I’m unable to retrieve error metrics from Prometheus, since all internal_write_errors,
internal_write_metrics_dropped and internal_write_metrics_rejected show zero results.

Is there any other metric that I should be using to retrieve output plugins errors?