Remove span name from stability guarantees

[lmolkova]

Changes

The context: we'd like to update span name for HTTP client spans to include HTTP route/operation name when it's available. Today span name includes only method (e.g. GET and could instead become more useful - GET users/{user_id}). See https://github.com/open-telemetry/semantic-conventions/pull/675 and https://github.com/open-telemetry/semantic-conventions/issues/923 for more details.

However, even in the general case, span name is not something dashboards and alerts should rely on:

• span name is usually constructed from attributes that would be more reliable for queries, dashboards and alerts
• span names can change depending on the available level of details (e.g. HTTP server span name looks like {method} {route} and route availability depends on the web framework).
• span name format, case sensitivity could be hard to enforce and may depend on the language.

Span name should be fine in queries like "top N longest" or "top N with highest error rate" - then, when span names become more descriptive, such queries would produce even better results.

• [x] Related issues https://github.com/open-telemetry/semantic-conventions/pull/675, https://github.com/open-telemetry/semantic-conventions/issues/923
• Related OTEP(s)
• Links to the prototypes (when adding or changing features)
• [ ] CHANGELOG.md file updated for non-trivial changes
• spec-compliance-matrix.md updated if necessary

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