Google Monitoring API filters support <,<=,>,>= operators as described
in its documentation:
https://cloud.google.com/monitoring/api/v3/filters.

Current version of the library only has mappings of:

1. ’{key}_prefix:{value}' -> '{key} = starts_with("{value}")'
2. ’{key}_suffix:{value}' -> '{key} = ends_with("{value}")'

This patch introduces additional mappings of:

3. ’{key}_greater:{value}' -> '{key} > {value}'
4. ’{key}_greaterequal:{value}' -> '{key} >= {value}'
5. ’{key}_less:{value}' -> '{key} < {value}'
6. ’{key}_lessequal:{value}' -> '{key} <= {value}'

So that it is possible, for example, to filter based on the
response_code values range, as shown below:

metric.type = "appengine.googleapis.com/http/server/response_count"
    AND metric.label.response_code < 600
    AND metric.label.response_code >= 500

This patch will allow to remove “hack” in the following tool:
https://github.com/odin-public/gcpmetrics that enables
queries to the monitoring api from the command line.

Fixed `tox -e cover` to have 100% coverage after proposed patch

1. Added docstring for select_metrics with the explanation of
additional endings. No reason to do the same for select_resources,
because all of the currently defined monitored resource labels are
strings.
2. Removed array->tuple conversion, moved constant directly into the if
condition.