Slo-generator

Fixes
* Partition table for BigQuery database.

Fixes
* Frequent 500s in Stackdriver timeseries.write

Fixes
* Add unit tests for ElasticSearch
* Add unit tests for Prometheus
* Improve overall documentation
* Add unit tests for Stackdriver Service Monitoring

Features
* Add Stackdriver Service Monitoring backend
* Reformat SLO reporting in a new dataclass
* Add working samples for each backend
* Reformat kwargs blocks in each backend to be less generic

Fixes
* Add Pylint tests and make them all pass
* Add test fixtures for Prometheus metrics backend

Features
* Add support for Prometheus metrics backend
* Add support for ElasticSearch metrics backend
* Add feature to support SLO config folder instead of single file
* Add feature to support parsing environmental variables from YAML file
* Add support for filter_valid in Stackdriver backend
* Rewrite documentation

Features
* feat: Basic SLO monitoring with Stackdriver