Optimizers: attribute based logarithm scale function should be "log1p" instead of "log"

[rbayet]

With a scale factor of 1, using an attribute based search optimizer with "logarithmic" scale function reduces the fulltext score of products to 0.
That is because the "missing" value is "1 / $scaleFactor", the field_value_factor with the "log" function ends up being 1 * log(1) (scaleFactor * log(fieldValue)) :

    public function getFunction(ContainerConfigurationInterface $containerConfiguration, OptimizerInterface $optimizer)
    {
        $field       = $this->getField($containerConfiguration, $optimizer);
        $scaleFactor = (float) $optimizer->getConfig('scale_factor');

        $function = [
            'field_value_factor' => [
                'field'    => $field,
                'factor'   => $scaleFactor,
                'modifier' => $optimizer->getConfig('scale_function'),
                'missing'  => 1 / $scaleFactor,
            ],
            'filter' => $optimizer->getRuleCondition()->getSearchQuery(),
        ];

        return $function;
    }

Since log(1) = 0 and the boost function is applied in multiply mode, it multiplies the fulltext _score to 0 :

• the products with an attribute value > 2 are correctly sorted / boosted
• the products without a value of with a value of 1 all have a _score of 0 and so are no longer sorted according to their base fulltext relevancy

Preconditions

Magento Version : EE 2.3.x

ElasticSuite Version : 2.8.x

Environment : Developer/Production

Third party modules : N/A

Steps to reproduce

1. Create a numeric attribute and add it to a type of products returned by a well-known search query generating more than 1 page of search results
2. Observe the order of search results and pick 3 products on the first page, give them a value of respectively 300, 20 and 1 for the numeric attribute
3. Create an attribute based search optimizer with a scale factor of 1 and the "logarithmic" scale function, enable it
4. Perform again the fulltext search

Expected result

1. The 3 products should move up in the search results page and the other products relative position should only impacted by that

Actual result

1. The products with a value of 300 and 20 are indeed boosted and put in the first positions
2. The product with a value of 1 does not seem to be boosted
3. All the other search results are no longer those expected, seemingly displayed at random (actually: by their product id ascending)