feat: add additionalLabels to serviceMonitor

[bradmccoydev]

Closes #364

📑 Description

This PR is to add the ability to add your own labels to the service monitor. In my case I use the Prometheus operator therefore I need to add an additional label so it references my prometheus release which is:

labels:
  release: kube-prometheus-stack

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

For the reviewer testing this, you can run these tests.

Scenario 1: Run helm chart with service monitor enabled is false

in the values.yaml, the serviceMonitor enabled is false as follows:

serviceMonitor:
  enabled: false

Scenario 2: Run helm chart without the additional labels

in the values.yaml, only use the serviceMonitor enable is true as follows:

serviceMonitor:
  enabled: true

Scenario 3: Run helm chart with the additional labels

in the values.yaml, only use the serviceMonitor enable is true and add labels as follows:

serviceMonitor:
  enabled: true
  additionalLabels:
    release: kube-prometheus-stack

To test the values run this command in the root directory:

helm install ./chart/operator--dry-run --devel --generate-name > manifests.yaml

Then look in the manifests.yaml file and search throught it (remember to delete it)

[AlexsJones]

Thanks @bradmccoydev would you mind adding this to the README in the table of available helm values?