Document/warn about cardinality issue with attributes where input "comes from the user"

[joaopgrassi]

During discussion about semconv and GDPR in #128, @pellared also brought the point that certain attributes are prone for metric cardinality issues.

Today, we don't have any guidance or recommendation about such danger. During the semconv wg meeting on 2023-11-31, it was discussed that it may be a good idea for us to add some text highlighting such issues so users are aware.

[trask]

Today, we don't have any guidance or recommendation about such danger. During the semconv wg meeting on 2023-11031, it was discussed that it may be a good idea for us to add some text highlighting such issues so users are aware.

For users, we have (some) warnings about such danger, e.g.

semantic-conventions/docs/http/http-metrics.md

Lines 131 to 133 in 0de415c

	> **Warning**
	> Since this attribute is based on HTTP headers, opting in to it may allow an attacker
	> to trigger cardinality limits, degrading the usefulness of the metric.