Remove the "MUST" wording which requires exemplar to be on-by-default

[reyang]

Fixes #2118, which is considered as a blocking issue for metrics SDK spec stable release.

Changes

We've discussed this during the 3/11/2022 metrics issue triage meeting.

The priority is to ship the stable version of the Metrics SDK spec, however the Exemplar part still has many open questions and there are not sufficient prototype from language SIGs. Here are some decisions we've made in order to move the needles:

• Instead of being blocked by the Exemplar spec, we will focus on shipping the SDK spec as Mixed, by moving most parts to Stable but leave Exemplar as "Feature-freeze" (PR Move metrics SDK spec to Mixed #2304).
• Weaken the wording in the Exemplar spec, so it won't cause conflict or introduce breaking change to the spec.

This PR removed the "exemplar must be on by default" wording, and used "should be turned off by default" instead. This gives the spec certain room, so later we can change it to "may be turned on by default" or "should be turned on by default" without having to bump the major version of the spec.

[jmacd]

I approve this PR with or without the attached, non-blocking suggestion.

[cijothomas]

the explanation/rationale for this change looks reasonable to me.

[jack-berg]

Approving, but I think exemplars are a really important feature we should strive to stabilize and enable by default in the near future. Want to reiterate this question:

I'd still like to understand criteria for marking Exemplar stable. Specifically, what more do we need to see, what issues exist in the current specification.