Function build_metric_lines 

fn build_metric_lines(
    entry: &Value,
    namespace: &str,
    constant_tags: &[String],
    include_path: bool,
    include_method: bool,
) -> Vec<String>

Builds the DogStatsD metric lines for one featherbit log entry.

Rendered as namespace.metric:value|type|#tag,tag. Factored out of the socket path so metric formatting is unit-testable.