Create a static metric using local.file

Grafana Alloy
1

I have a file on a Linux filesystem that contains a timestamp. I would like to take that timestamp and create a metric with it that can then be used in Grafana.

Is it possible to create a static metric with Alloy? And then have that metric use the timestamp as a label or even the value?

This is what I have been trying so far:

local.file "update_timestamp" {
  filename  = "/var/log/update-timestamp"
}

prometheus.scrape "update_timestamp" {
  targets = [{
      "__address__" = "alloy.internal:12345",
      "timestamp" = local.file.update_timestamp.content,
      "__fake_name__" = "placeholder",
    }]
  forward_to = [prometheus.relabel.update_timestamp.receiver]
}

prometheus.relabel "update_timestamp" {
  rule {
    source_labels = ["__fake_name__"]
    regex = "placeholder"
    action = "keep"
  }
  rule {
    source_labels = ["timestamp"]
    regex = "([0-9]+)"
    target_label = "__value__"
    replacement = "$1"
  }
  rule {
    target_label = "__name__"
    replacement = "update_timestamp"
  }
  forward_to = [prometheus.remote_write.default.receiver]
}

Alloy debug shows all the metrics with the timestamp label, for example:

{
  __name__="alloy_build_info",
  branch="HEAD",
  goarch="amd64",
  goos="linux",
  goversion="go1.24.3",
  instance="alloy.internal:12345",
  job="update_timestamp",
  revision="d3d7931",
  tags="netgo,builtinassets,promtail_journal_enabled",
  timestamp="{false 1749749967\n}",
  version="v1.9.1"
} => {}

But there is but no new metric that I can filter on. So it appears that the relabel never happens.

Is what I am trying to do even possible?

2

I find using stage.metrics in loki.process to be the easiest way to extract metrics from log files (see loki.process | Grafana Alloy documentation).

That said, I don’t think you can explicitly set a timestamp for the metrics.

3

The file is not technically a log file. It just happens to live in /var/log. The file only ever contains a single timestamp, and only a timestamp.

File: update-timestamp

1749749967

I will look into loki.process. Thanks for the recommendation.