Reduce redis key size by half for Prometheus::ProxyService

What does this MR do?

Reduce the size of the redis cache keys used by Prometheus::ProxyService by half

Reduce the refresh queries to Prometheus by half also.

Screenshots

Keys before (notice duplication at Environment:20 ):

"cache:gitlab:Environment:20:GET:query_range:{\"query\"=>\"sum(aws_elb_httpcode_backend_5_xx_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"}) / sum(aws_elb_request_count_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"})\", \"start\"=>\"2019-11-12T17:50:42.000Z\", \"end\"=>\"2019-11-13T01:50:42.000Z\", \"step\"=>\"60\"}:Environment:20:GET:query_range:{\"query\"=>\"sum(aws_elb_httpcode_backend_5_xx_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"}) / sum(aws_elb_request_count_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"})\", \"start\"=>\"2019-11-12T17:50:42.000Z\", \"end\"=>\"2019-11-13T01:50:42.000Z\", \"step\"=>\"60\"}"

Keys after:

"cache:gitlab:Environment:20:GET:query_range:{\"query\"=>\"sum(aws_elb_httpcode_backend_5_xx_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"}) / sum(aws_elb_request_count_sum{container_name!=\\\"POD\\\",environment=\\\"production\\\"})\", \"start\"=>\"2019-11-12T17:40:04.000Z\", \"end\"=>\"2019-11-13T01:40:04.000Z\", \"step\"=>\"60\"}"

Keys before and after can be measured with a command like:

redis-cli -s /Users/dwilkins/source/gitlab/gdk-demo/redis/redis.socket KEYS '*query_range*'

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team