Change spanmetrics metric names and labels to match OTel conventions

[mapno]

What this PR does:

Changes spanmetric's metric names and labels to match OTel's spanmetrics spec.

Which issue(s) this PR fixes:
Fixes #1444

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[kvrhdn]

Oh just a though, should we add a list of changes to the changelog so it's easy for people to find out what metrics/labels to change?

Something like this list: grafana/grafana#48654 (comment)

[joe-elliott]

Agree with adding the metric name changes to the changelog. We often add details like that:

tempo/CHANGELOG.md

Lines 53 to 76 in 02a945d

	* [ENHANCEMENT] Add hedging to queries to external endpoints. [#1350](https://github.com/grafana/tempo/pull/1350) (@joe-elliott)
	New config options and defaults:
	```
	querier:
	search:
	external_hedge_requests_at: 5s
	external_hedge_requests_up_to: 3
	```
	** BREAKING CHANGE **
	Querier options related to search have moved under a `search` block:
	```
	querier:
	search_query_timeout: 30s
	search_external_endpoints: []
	search_prefer_self: 2
	```
	becomes
	```
	querier:
	search:
	query_timeout: 30s
	prefer_self: 2
	external_endpoints: []
	```