metrics: increase prometheus data retention

[cirocosta]

By default, the prometheus Chart configures a retention policy that's limited to
15 days.

## Prometheus data retention period (default if not specified is 15 days)
##
retention: "15d"

(https://github.com/helm/charts/blob/67ed74b614bb5f4e068017101a8673c63459f383/stable/prometheus/values.yaml#L841-L843)

(the change would be somewhere here:

hush-house/deployments/with-creds/metrics/values.yaml

Line 24 in a14d083

server:

)

While that's been in our favor when it comes to having to have a very small disk
(30GB), the price of not having the data for longer than just 15d turns out to
be bigger (when, for instance, comparing how Concourse performed over a longer
period of time).

I'm not entirely sure of what a good number for this is, but, with the
intention of shipping Concourse on a 3-week basis, it seems reasonable to me to
have at least 6 weeks of retention (so that we have the data for the entire
period between a deploy and another.

Thanks!

[cirocosta]

#101