Snuba-sdk

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- Fix a bug where the groupby on the MetricsQuery was not being serialized

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- Bug fixes and changes to support using the MetricsQuery in production
- Add `granularity` to the Rollup to delineate the interval (the results
bucketing) vs. the granularity of the underlying data
- `granularity` is mandatory for now
- This means adding back the restrictions of having interval or totals but not both
- Use the interval to rollup the time automatically
- This also means supporting arbitrary intervals
- Automatically order timeseries by the rolled up time
- Allow AliasedExpression in the groupby clause. This is to support resolving
in Sentry. The result of the query should have the original tag key, not the
resolved tag number. With AliasedExpression we can have Clickhouse do the mapping
automatically.

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- Add validators and translators for the MetricsQuery and all its child objects
- Rename some components to be consistent between Metric and Metrics
- Move MetricsScope and Rollup to the timeseries module
- Also stop treating them as Expressions, so they can be visited in a more nuanced way

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Various fixes & improvements

- add additional setters to metric query child classes (119) by enochtangg
- update contributing to reflect new release procedure (118) by evanh

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- Add a new syntax for querying timeseries metrics
- Add a MetricsQuery class that can be attached to a Request
- Add a Timeseries class that is used to capture a simple timeseries query
- Add various other classes to support the new syntax

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- `tenant_ids` required field added previously is now optional to support gradual adoption