Add support for Query Range String: Change step to string and support standard time.Duration parsing

[nehayward]

What this PR does / why we need it:
This changes Query Range to a string to support step changes.

Which issue(s) this PR fixes:
Fixes #1209

Special notes for your reviewer:
This still needs some refinement and I wanted to get some eyes on it from the Loki community before moving forward with the changes.

Checklist

• Documentation added
• Tests updated -- (in progress)

[CLAassistant]

All committers have signed the CLA.

[cyriltovena]

Thanks a lot for this, don’t forget we need to support both format.

See https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries

step=<duration | float>: Query resolution step width in duration format or float number of seconds

[nehayward]

Any suggestion on how you'd like to do that. Should add another optional arg

[cyriltovena]

No the same argument just try converting to integer if it fails try parsing a duration if it fails it fails ;)

[cyriltovena]

@nehayward any news, I might need this too.

[nehayward]

Let me update my PR.

[ghost]

It seems loki still doesn't support float numbers. I got error when I run query via Grafana.

rpc error: code = Code(400) desc = strconv.Atoi: parsing "54.982": invalid syntax

I'm using the latest docker image for both grafana and Loki.
For reference:
`
version: "3"

networks:
loki:

services:
loki:
image: grafana/loki:v1.0.0
ports:
- "3100:3100"
volumes:
- /data/loki/conf:/data/loki/conf
command: -config.file=/data/loki/conf/loki.yaml
networks:
- loki

grafana:
image: grafana/grafana:master
ports:
- "3000:3000"
networks:
- loki
`

[cyriltovena]

yes @nehayward do you want me to take over ?

[cyriltovena]

Fixed.