CLI flags as environment variables

Description

Solves issue https://github.com/timescale/prometheus-postgresql-adapter/issues/43. In general, to make the app more compliant with the 12Factor standard (https://12factor.net/config), and for security reasons mentioned in the issue, all CLI flags have an equivalent environment variable.

Motivation and Context

This is a feature enhancement. Initially I was working on a Helm chart for this app (https://github.com/helm/charts/pull/7927), and I thought it would be better to have environment variable support. I will probably resume work on the Helm chart, but the maintainers of that repo are slow as hell; probably better if Timescale had its own repository for Helm charts to be honest.

I'm not sure if this qualifies as a breaking change, but the CLI flags should be renamed to not use dots (.) in them. See my comment here (https://github.com/timescale/prometheus-postgresql-adapter/issues/43#issuecomment-481879131), but in short, the BASH shell doesn't allow for dots in environment variables names when you run export.

How Has This Been Tested?

Locally ran the build command:

GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -a -installsuffix cgo --ldflags '-w' -o prometheus-postgresql-adapter

And then ran docker-compose up --detach --build with some environment variables mapped to the container. I can confirm that the prometheus-postgresql-adapter successfully reads the environment variables injected.

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[X] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

[X] My code follows the code style of this project.
[X] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.

timescale / prometheus-postgresql-adapter