feat: add OTLP gRPC exporter

[seuros]

Added Opentelemetry to the gRPC exporter , this mirror the HTTP example.

[GlenDC]

Some initial feedback. Once this is handled we can start looking into details. Even though I must say beyond this feedback it does look good. So don't think there will be much left to do (if anything at all) after we resolved these matters.

[GlenDC]

there should be no need for you to import opentelemetry or opentelemetry-sdk directly. Both are afaik already part of rama-core and can be used via there.

opentelemetry-proto should be dropped (see other comment).

[GlenDC]

this can be replaced with compiling grpc protos

[GlenDC]

There should be no need to watch these env variables btw we already watch proto folder, should be enough for this, this is just about building the code from proto so no need to check env variables

[GlenDC]

Let me know @seuros if you need any guidance or help with this one.
Will be good to have it as it will mean we can remove tonic and tonic-prost from the dep tree:

% cargo tree --all-features -i tonic
tonic v0.14.4
├── opentelemetry-proto v0.31.0
│   └── opentelemetry-otlp v0.31.0
│       └── rama v0.3.0-rc1 (/Users/glendc/code/github.com/plabayo/rama)
└── tonic-prost v0.14.4
    └── opentelemetry-proto v0.31.0 (*)