otel: Add trace sampling, add small fixed

[javorszky]

Adds code to rust to take a trace sampling ratio, and adds code on the Unit side implementing the settings/telemetry/sample_ratio configuration path that will be resolved into a number. Adds a validator that makes sure that the sample ratio is in the range [0, 1].

Also fixes the following nits:

• no need to have the "send" function be async or wrapped into tokio that C calls
• replace a type inference from _ to usize because we know that's the only thing it can (should) be
• break out a time duration into a const rather than creating them twice

Signed-off-by: Gabor Javorszky g.javorszky@f5.com

[javorszky]

Manual tests I've done:

• it builds
• settings/telemetry/sample_ratio configuration option is accepted
• when configured to use grpc, traces do end up in grafana