Using Promtail to sum log line values - Pipeline Stages - Metrics

Question

Given a log lines such as:

[Some info] myproject - number of values: 50
[Some info] myproject - number of values: 100
[Some info] myproject - number of values: 20

I am looking to add these values up and display them in Grafana. i.e. given a 10m interval, Grafana will display 'myproject' had a total of 170 values.

I've been looking at https://grafana.com/docs/loki/latest/clients/promtail/stages/metrics/#gauge which seems to do just that, and here is my config within Promtail:

server:
  http_listen_port: 9080

positions:
  filename: /tmp/positions.yaml

clients:
  - url: http://<host>:3100/loki/api/v1/push

scrape_configs:
- job_name: system
  static_configs:
  - targets:
      - localhost
    labels:
      job: nifi_server_1
      __path__: /var/log/nifi*
  pipeline_stages:
  - regex:
      expression: 'number of values: (?P<numValues>[0-9]{1,4})'
  - labels:
      numOfvalues: numValues
  - metrics:
      number_of_values:
        type: Gauge
        description: "sum of number of values"
        source: numValues
        config:
          action: add

However I end up with strange results where Grafana things each individual 'value' is a label in itself, rather than adding them up: