this is a sample script that clean old metrics from pushgateway, not very clean but working anyway, used as sample in a issue

purge_prom_pushgateway.sh

#!/usr/bin/env bash

trap 'echo "got sigterm" ; exit 0' SIGTERM

EXPIRATION_SECONDS=${EXPIRATION_SECONDS:-900}
PGW_URL=${PGW_URL:-http://127.0.0.1:9091}

function convert_to_standardnotation() {
    # convert number from scientific notation to standard( ie  '1.5383780136826127e+09' )
    echo $1 | awk '{printf("%.0f", $1)}'
}

function extract_pushgateway_variable() {
    local -r _METRIC=$1
    local -r _VARNAME=$2
    #echo 'push_time_seconds{instance="10.32.32.7",job="bk_jenkins"} 1.5383802210997093e+09' | sed -r 's/.*instance="([^"]*).*/\1/g'
    echo $_METRIC | sed -r "s/.*${_VARNAME}=\"([^\"]*).*/\\1/g"
    # sample usage :
    # extract_pushgateway_variable 'push_time_seconds{instance="10.32.32.7",job="bk_jenkins"} 1.5383802210997093e+09' 'instance'
}

function check_metric_line() {
    local -r _line=$1
    METRIC_TIME=$(echo $_line | awk '{print $2}')
    #echo "mtime = $_line -> $METRIC_TIME "
    METRIC_TIME=$(convert_to_standardnotation $METRIC_TIME)
    #echo "$CURRENT_TIME - $METRIC_TIME "
    METRIC_AGE_SECONDS=$((CURRENT_TIME - METRIC_TIME))
    if [ "$METRIC_AGE_SECONDS" -gt "$EXPIRATION_SECONDS" ]; then
        metricInstance=$(extract_pushgateway_variable "$_line" 'instance')
        metricJob=$(extract_pushgateway_variable "$_line" 'job')
        echo "[INFO] job should be deleted $metricJob  - $metricInstance  age: $METRIC_AGE_SECONDS "
        curl -s -X DELETE "$PGW_URL/metrics/job/${metricJob}/instance/${metricInstance}"
    fi
}

function check_expired_metric_loop() {
    export CURRENT_TIME=$(date +%s)
    METRICS_LIST=$(curl -s $PGW_URL/metrics | egrep "^push_time_seconds")
    echo "$METRICS_LIST" | while read -r line || [[ -n "$line" ]]; do
        check_metric_line "$line"
    done
}

while :; do
    check_expired_metric_loop
    sleep $((EXPIRATION_SECONDS / 3))
done