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

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

EXPIRATION_SECONDS=${EXPIRATION_SECONDS:-900}
PGW_URL=${PGW_URL:-http://pushgateway}

function convert_to_standardnotation(){
    # convert number from scientific notation to standar d( ie  '1.5383780136826127e+09' )
    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
sleep $((EXPIRATION_SECONDS / 3 ))

}
while : ; do
check_expired_metric_loop
done 

The function "convert_to_standardnotation" did not work on one of my servers, I fixed it like this:

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

Thanks . It is worked as expected.