SDK should collect OTel metrics by default

[cedricziel]

Is your feature request related to a problem?
Currently, tracing is the most mature signal we use in OTel-PHP. There are other use-cases where metrics are a desired outcome for users to use with analytics backends.

Describe the solution you'd like

Support the canonical metrics:

• HTTP metrics
• Database metrics
• Messaging metrics
• RPC/gRPC metrics

Describe alternatives you've considered
None

Additional context

Some of the metrics are not yet fully specified, but we should probably define first how they are best collected (for example SDK vs extension).