ENH: Add length attribute to Interval and IntervalIndex
- [X] closes #18789
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=h1) Report
> Merging [#18805](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/316acbf065f5d1e30fdc7fcced3a53a2f4b14655?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18805/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18805      +/-   ##
==========================================
+ Coverage   91.65%   91.67%   +0.02%     
==========================================
  Files         154      154              
  Lines       51368    51373       +5     
==========================================
+ Hits        47080    47095      +15     
+ Misses       4288     4278      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.54% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.84% <12.5%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/18805/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.88% <100%> (+0.05%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18805/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.68% <0%> (-0.22%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18805/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=footer). Last update [316acbf...6a719ef](https://codecov.io/gh/pandas-dev/pandas/pull/18805?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you make sure this is in api.rst (though IIRC we are listing these attributes directly in the class). @jreback : added this to api.rst, and made minor changes to the `IntervalIndex` docstring.  Generated the docs and changes look good.   thanks!