Add support for the comparison filters of Google Monitoring API

[maxkuzkin]

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:

1. ’{key}_greater:{value}' -> '{key} > {value}'
2. ’{key}_greaterequal:{value}' -> '{key} >= {value}'
3. ’{key}_less:{value}' -> '{key} < {value}'
4. ’{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.

[rimey]

I would suggest inserting something like the following in the docstring for select_metrics at Line 315, immediately after the explanation of _prefix and _suffix. I don't think we need to say anything in the docstring for select_resources, because all of the currently defined monitored resource labels are strings.

        If the label's value type is ``INT64``, a similar notation can be
        used to express inequalities:

        ``<label>_less=<value>`` generates::

            metric.label.<label> < <value>

        ``<label>_lessequal=<value>`` generates::

            metric.label.<label> <= <value>

        ``<label>_greater=<value>`` generates::

            metric.label.<label> > <value>

        ``<label>_greaterequal=<value>`` generates::

            metric.label.<label> >= <value>

[rimey]

I made just a couple of comments. Thank you for the contribution!

[maxkuzkin]

Hi Ken, updated PR now has improvements you've suggested.

[rimey]

LGTM