Add blog post about new sensor state classes (#1023)

* Add blog post about new sensor state classes

* Update blog/2021-08-12-state_class_total.md

Co-authored-by: Joakim Sørensen <hi@ludeeus.dev>

* Attempt to fix tables

* Apply suggestions from code review

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>
Co-authored-by: Tom Brien <TomBrien@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Franck Nijhof <git@frenck.dev>

* Tweak

* Update 2021-08-12-state_class_total.md

* Bump date of the blogpost

Co-authored-by: Joakim Sørensen <hi@ludeeus.dev>
Co-authored-by: Martin Hjelmare <marhje52@gmail.com>
Co-authored-by: Tom Brien <TomBrien@users.noreply.github.com>
Co-authored-by: Franck Nijhof <git@frenck.dev>

blog/2021-08-16-state_class_total.md

@@ -0,0 +1,85 @@
+---
+author: Erik Montnémery
+authorURL: https://github.com/emontnemery
+title: "New sensor state classes: total and total_increasing"
+---
+
+Two new state classes, `STATE_CLASS_TOTAL` and `STATE_CLASS_TOTAL_INCREASING` have been
+added. In addition, it is no longer mandatory to set the `last_reset` attribute for
+`sum` statistics to be generated. The driver for the changes is to make it easier to
+integrate with devices, like utility meters.
+
+### State classes
+
+There are now 3 defined state classes:
+
+- `STATE_CLASS_MEASUREMENT`, the state represents a measurement in present time, for 
+   example a temperature, electric power, etc. For supported sensors, statistics of 
+   hourly min, max and average sensor readings are compiled.
+- `STATE_CLASS_TOTAL`, the state represents a total amount that can both increase and
+   decrease, e.g. the value of a stock portfolio. When supported, the accumulated growth
+   or decline of the sensor's value since it was first added is updated hourly.
+- `STATE_CLASS_TOTAL_INCREASING`, a monotonically increasing total, e.g. an amount of
+   consumed gas, water or energy. When supported, the accumulated growth of the sensor's
+   value since it was first added is updated hourly.
+
+#### `STATE_CLASS_MEASUREMENT`
+
+For sensors with state_class `STATE_CLASS_MEASUREMENT`, it is deprecated to set the
+`last_reset` attribute, and it will be ignored in Home Assistant 2021.10.
+
+#### `STATE_CLASS_TOTAL`
+
+For sensors with state_class `STATE_CLASS_TOTAL`, the `last_reset` attribute can
+optionally be set to gain manual control of meter cycles; each time last_reset changes
+the corresponding value is used as the zero-point when calculating `sum` statistics.
+If last_reset is not set, the sensor's value when it was first added is used as the
+zero-point when calculating `sum` statistics.
+
+Example of `STATE_CLASS_TOTAL` without last_reset:
+
+| t                      | state  | sum    |
+| :--------------------- | -----: | -----: |
+|   2021-08-01T13:00:00  |  1000  |     0  |
+|   2021-08-01T14:00:00  |  1010  |    10  |
+|   2021-08-01T15:00:00  |     0  | -1000  |
+|   2021-08-01T16:00:00  |     5  |  -995  |
+
+Example of `STATE_CLASS_TOTAL` with last_reset:
+
+| t                      | state  | last_reset          | sum    |
+| :--------------------- | -----: | ------------------- | -----: |
+|   2021-08-01T13:00:00  |  1000  | 2021-08-01T13:00:00 |     0  |
+|   2021-08-01T14:00:00  |  1010  | 2021-08-01T13:00:00 |    10  |
+|   2021-08-01T15:00:00  |  1005  | 2021-08-01T13:00:00 |     5  |
+|   2021-08-01T16:00:00  |     0  | 2021-09-01T16:00:00 |     5  |
+|   2021-08-01T17:00:00  |     5  | 2021-09-01T16:00:00 |    10  |
+
+
+#### `STATE_CLASS_TOTAL_INCREASING`
+
+For sensors with state_class `STATE_CLASS_TOTAL_INCREASING`, a decreasing value is
+interpreted as the start of a new meter cycle or the replacement of the meter. It is
+important that the integration ensures that the value cannot erroneously decrease in 
+the case of calculating a value from a sensor with measurement noise present. The
+last_reset attribute will be ignored when compiling statistics. This state class is
+useful for gas meters, electricity meters, water meters etc. The value when the sensor
+reading decreases will be used as zero-point when calculating `sum` statistics.
+
+Example of `STATE_CLASS_TOTAL_INCREASING`:
+
+| t                      | state  | sum  |
+| :--------------------- | -----: | ---: |
+|   2021-08-01T13:00:00  |  1000  |   0  |
+|   2021-08-01T14:00:00  |  1010  |  10  |
+|   2021-08-01T15:00:00  |     0  |  10  |
+|   2021-08-01T16:00:00  |     5  |  15  |
+
+Example of `STATE_CLASS_TOTAL_INCREASING` where the sensor does not reset to 0:
+
+| t                      | state  | sum  |
+| :--------------------- | -----: | ---: |
+|   2021-08-01T13:00:00  |  1000  |   0  |
+|   2021-08-01T14:00:00  |  1010  |  10  |
+|   2021-08-01T15:00:00  |     5  |  10  |
+|   2021-08-01T16:00:00  |     10 |  15  |