# Shell text defining `otlp_gauges`, which pushes gauges to this host's OTLP/HTTP
# metrics receiver:
#
#   otlp_gauges <service.name> <attr=value,attr=value> <metric=value>...
#
# One resource, one set of data-point attributes, one data point per metric.
# Values are numbers. A failed build of the payload or a failed push prints a
# warning on stderr and returns 0, so a unit's own result never depends on
# telemetry. The swarm collector stamps `hive` on every sample it receives from
# a hive, so callers do not set it.
#
# `endpoint` is the receiver's base URL, or null on a host without one, where
# the function does nothing.
#
# Pure function, NOT a NixOS module. Needs `jq`, `date` and `curl` on the
# unit's `path`.
{ lib }:
{ endpoint }:
if endpoint == null then
  ''
    otlp_gauges() { :; }
  ''
else
  ''
    otlp_gauges() {
      local service="$1" attrs="$2" body
      shift 2
      body="$(jq -n -c --arg service "$service" --arg attrs "$attrs" --arg now "$(date +%s%N)" '
        ($attrs | split(",") | map(select(length > 0) | index("=") as $i
          | {key: .[:$i], value: {stringValue: .[$i + 1:]}})) as $attributes
        | {resourceMetrics: [{
            resource: {attributes: [{key: "service.name", value: {stringValue: $service}}]},
            scopeMetrics: [{metrics: ($ARGS.positional | map(index("=") as $i
              | {name: .[:$i], gauge: {dataPoints: [{
                  asDouble: (.[$i + 1:] | tonumber),
                  attributes: $attributes,
                  timeUnixNano: $now}]}}))}]}]}
        ' --args "$@")" || {
        echo "warning: could not build the metrics payload for $service: $*" >&2
        return 0
      }
      printf '%s' "$body" |
        curl --fail --silent --show-error --max-time 10 \
          -H 'Content-Type: application/json' --data-binary @- \
          ${lib.escapeShellArg "${endpoint}/v1/metrics"} ||
        echo "warning: could not push metrics for $service" >&2
      return 0
    }
  ''