Sensor filter to round to significant digits

[gapple]

What does this implement/fix?

Add a filter to round sensor values to a configured number of significant digits.

Useful for sensors like ambient light which use variable measurement interval and gain to cover a wide value range (0.0072 to 120000 lx), and the current round or round_to_multiple filters aren't effective.

Original Value	round_to_sigfig: 2	round_to_sigfig: 4
123456.789	120000	123500
123.4567	120	123.5
12.3456	12	12.35
1.23456	1.2	1.235
0.123456	0.12	0.1235

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code quality improvements to existing code or addition of tests
• Other

Related issue or feature (if applicable):

• fixes

Pull request in esphome-docs with documentation (if applicable):

• Round to significant figures filter esphome-docs#5485

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx
• nRF52840

Example entry for config.yaml:

sensor:
  - platform: veml7700
    address: 0x10
    accuracy_decimals: 0
    ambient_light: 
      name: "Ambient light"
      filters:
        - sliding_window_moving_average:
            window_size: 5
            send_every: 5
        - round_to_sigfig: 2

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[github-actions]

To use the changes from this PR as an external component, add the following to your ESPHome configuration YAML file:

external_components:
  - source: github://pr#11157
    components: [const, sensor]
    refresh: 1h

---

(Added by the PR bot)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.71%. Comparing base (c38c2a1) to head (e5c19b7).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #11157      +/-   ##
==========================================
- Coverage   71.74%   71.71%   -0.03%     
==========================================
  Files          53       53              
  Lines       11031    11031              
  Branches     1482     1482              
==========================================
- Hits         7914     7911       -3     
- Misses       2719     2721       +2     
- Partials      398      399       +1     

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[github-actions]

👋 Hi there! I've automatically requested reviews from codeowners based on the files changed in this PR.

@clydebarrow - You've been requested to review this PR as codeowner(s) of 5 file(s) that were modified. Thanks for your time! 🙏

[clydebarrow]

1. You would need docs and tests. Best mark as draft until those are done.
2. In the absence of docs, it's not obvious what this does and why the existing round filter doesn't suffice. Some examples would help.

[clydebarrow]

Not sure this is the best name for the filter - "fig" doesn't gel with the "digits" used in the config, and generally speaking abbreviating words in config options is not ideal. Maybe "maximum_significant_digits"? It's wordy, but clear, and eliminates the need to have "digits" as a sub-key.

Suggested change

	"round_to_sigfig",
	RoundSigfigFilter,
	cv.maybe_simple_value(
	{
	cv.Required(CONF_DIGITS): cv.positive_not_null_int,
	},
	key=CONF_DIGITS,
	),
	)
	async def round_sigfig_filter_to_code(config, filter_id):
	return cg.new_Pvariable(
	filter_id,
	config[CONF_DIGITS],
	)
	"maximum_significant_digits",
	RoundSigfigFilter,
	cv.positive_not_null_int,
	)
	async def round_sigfig_filter_to_code(config, filter_id):
	return cg.new_Pvariable(
	filter_id,
	config,
	)

[esphome]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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