OpenTelemetry tracer - Align attributes with OTel HTTP semantic conventions

*Title*: *Align Envoy span attributes with the OTel HTTP semantic conventions*

*Description*:

The OpenTelemetry semantic conventions for HTTP were marked stable in: https://github.com/open-telemetry/semantic-conventions/pull/377.

These conventions were adapted and reviewed by the community and subject-matter experts. As of today, the spans produced by Envoy are of course not aligned with the OTel conventions. This issue is to discuss if the Envoy community wants to align with them.

Pros:

- Standardization of attributes which provide users a consistent observability experience
- Observability back-ends can rely on such conventions, also giving a good and consistent experience for users

Cons:

- It is a breaking change for Envoy users. The existing attributes may be used for alerts, dashboards and etc
- Envoy records the attributes in a lot of places, and by changing those not only users using the OTel tracer will be impacted but users that use the other available tracers in Envoy, such as XRay, Datadog and etc.

To avoid the hard breaking change, maybe we can have some sort of "opt-in" configuration where users then opt in and Envoy starts producing spans aligned with the OTel HTTP semantic conventions. This allow them to gradually migrated to the new format.

[optional *Relevant Links*:]

- [OTel HTTP semantic conventions for spans](https://github.com/open-telemetry/semantic-conventions/blob/main/docs/http/http-spans.md)