[prometheus-postgres-exporter] Add support for `DATA_SOURCE_USER_FILE` environmental variable

[ubajze]

Is your feature request related to a problem ?

We use Vault dynamic database secrets, where both username and password are dynamically generated. We then use a Vault injector to inject a username and password into a pod as a file. I see that Postgres exporter supports the variable DATA_SOURCE_USER_FILE, where you specify the name of the file where the username is rendered. This is currently not supported in config:

helm-charts/charts/prometheus-postgres-exporter/values.yaml

Lines 126 to 155 in 62734a5

	config:
	## The datasource properties on config are passed through helm tpl function.
	## ref: https://helm.sh/docs/developing_charts/#using-the-tpl-function
	datasource:
	# Specify one of both datasource or datasourceSecret
	host: ''
	user: postgres
	userSecret: {}
	# Secret name
	# name:
	# User key inside secret
	# key:
	# Only one of password, passwordFile, passwordSecret and pgpassfile can be specified
	password:
	# Specify passwordFile if DB password is stored in a file.
	# For example, to use with vault-injector from Hashicorp
	passwordFile: ''
	# Specify passwordSecret if DB password is stored in secret.
	passwordSecret: {}
	# Secret name
	# name:
	# Password key inside secret
	# key:
	pgpassfile: ''
	# If pgpassfile is set, it is used to initialize the PGPASSFILE environment variable.
	# See https://www.postgresql.org/docs/14/libpq-pgpass.html for more info.
	port: "5432"
	database: ''
	sslmode: disable
	extraParams: ''

Describe the solution you'd like.

I want to specify the filename in a configuration. This is how the configuration should look like:

config:
  datasource:
    usernameFile: "/path/to/my/user/file"

The template would look similar to this:

helm-charts/charts/prometheus-postgres-exporter/templates/deployment.yaml

Lines 103 to 115 in 62734a5

	{{- if .Values.config.datasource.passwordFile }}
	- name: DATA_SOURCE_PASS_FILE
	value: {{ tpl .Values.config.datasource.passwordFile . }}
	{{- else }}
	- name: DATA_SOURCE_PASS
	valueFrom:
	secretKeyRef:
	{{- if .Values.config.datasource.passwordSecret }}
	name: {{ tpl .Values.config.datasource.passwordSecret.name . }}
	key: {{ tpl .Values.config.datasource.passwordSecret.key . }}
	{{- else }}
	name: {{ template "prometheus-postgres-exporter.fullname" . }}
	key: data_source_password

          {{- if .Values.config.datasource.usernameFile }}
          - name: DATA_SOURCE_USER_FILE
            value: {{ tpl .Values.config.datasource.usernameFile . }}
          {{- else }}
          - name: DATA_SOURCE_USER
            valueFrom:
              secretKeyRef:
          {{- if .Values.config.datasource.usernameSecret }}
                name: {{ tpl .Values.config.datasource.usernameSecret.name . }}
                key: {{ tpl .Values.config.datasource.usernameSecret.key . }}
          {{- else }}
                name: {{ template "prometheus-postgres-exporter.fullname" . }}
                key: data_source_password

Describe alternatives you've considered.

I can manually set the environmental variable:

extraEnvs:
  - name: "DATA_SOURCE_USER_FILE"
    value: "/path/to/my/user/file"

Additional context.

No response