Metrics: implement stable exporters

[inge4pres]

Describe the current state

The metrics SDK currently has a working implementation of in-memory exporter, and an incomplete support for OTLP exporter (see sdk/metrics/exporters/otlp.zig).

Desired state

It should support all the stable exporters listed in https://opentelemetry.io/docs/specs/otel/metrics/sdk_exporters/

• InMemory
• StdOut
• OTLP

Dev exporters are listed in #36

[inge4pres]

OTLP implementations is incomplete, re-opening.

[inge4pres]

We can close this with one more PR to make the stable exporters pub in sdk.zig and maybe e build an example for it.

[inge4pres]

Example added in #71