Refactor.

* Move metrics to another compilation unit.
* Add tests to metrics.
* Refactor builder to use `url` and `heartbeat` properties instead of
  ddtelemetry::config::Config.
* Add basic documentation.

data-pipeline/src/telemetry/metrics.rs

@@ -0,0 +1,100 @@
+// Copyright 2024-Present Datadog, Inc. https://www.datadoghq.com/
+// SPDX-License-Identifier: Apache-2.0
+
+use datadog_trace_utils::trace_utils::SendDataResult;
+use std::collections::HashMap;
+
+/// Structure to accumulate partial results coming from sending traces to the agent.
+#[derive(Default)]
+pub struct Metrics {
+    pub api_requests: u64,
+    pub api_responses_count_per_code: HashMap<u16, u64>,
+    pub api_errors_timeout: u64,
+    pub api_errors_network: u64,
+    pub api_errors_status_code: u64,
+    pub bytes_sent: u64,
+    pub chunks_sent: u64,
+    pub chunks_dropped: u64,
+}
+
+impl Metrics {
+    /// Updates the metric internal properties based on `result` contents.
+    pub fn update(&mut self, result: &SendDataResult) {
+        self.api_requests += result.requests_count;
+        self.api_errors_timeout += result.errors_timeout;
+        self.api_errors_network += result.errors_network;
+        self.api_errors_status_code += result.errors_status_code;
+        self.bytes_sent += result.bytes_sent;
+        self.chunks_sent += result.chunks_sent;
+        self.chunks_dropped += result.chunks_dropped;
+
+        for (status_code, count) in &result.responses_count_per_code {
+            *self
+                .api_responses_count_per_code
+                .entry(*status_code)
+                .or_default() += count;
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn update_test() {
+        let mut result = SendDataResult::default();
+        let mut metrics = Metrics::default();
+
+        assert_eq!(metrics.api_requests, 0);
+        assert_eq!(metrics.api_errors_timeout, 0);
+        assert_eq!(metrics.api_errors_network, 0);
+        assert_eq!(metrics.api_errors_status_code, 0);
+        assert_eq!(metrics.bytes_sent, 0);
+        assert_eq!(metrics.chunks_dropped, 0);
+        assert!(metrics.api_responses_count_per_code.is_empty());
+
+        metrics.update(&result);
+        assert_eq!(metrics.api_requests, 0);
+        assert_eq!(metrics.api_errors_timeout, 0);
+        assert_eq!(metrics.api_errors_network, 0);
+        assert_eq!(metrics.api_errors_status_code, 0);
+        assert_eq!(metrics.bytes_sent, 0);
+        assert_eq!(metrics.chunks_dropped, 0);
+        assert!(metrics.api_responses_count_per_code.is_empty());
+
+        result.requests_count = 1;
+        result.chunks_dropped = 1;
+        result.bytes_sent = 1;
+        result.errors_timeout = 1;
+        result.errors_network = 1;
+        result.errors_status_code = 1;
+        result.responses_count_per_code.insert(200, 1);
+
+        metrics.update(&result);
+        assert_eq!(metrics.api_requests, 1);
+        assert_eq!(metrics.api_errors_timeout, 1);
+        assert_eq!(metrics.api_errors_network, 1);
+        assert_eq!(metrics.api_errors_status_code, 1);
+        assert_eq!(metrics.bytes_sent, 1);
+        assert_eq!(metrics.chunks_dropped, 1);
+        assert_eq!(metrics.api_responses_count_per_code.len(), 1);
+        assert_eq!(
+            *metrics.api_responses_count_per_code.get(&200_u16).unwrap(),
+            1
+        );
+
+        metrics.update(&result);
+        assert_eq!(metrics.api_requests, 2);
+        assert_eq!(metrics.api_errors_timeout, 2);
+        assert_eq!(metrics.api_errors_network, 2);
+        assert_eq!(metrics.api_errors_status_code, 2);
+        assert_eq!(metrics.bytes_sent, 2);
+        assert_eq!(metrics.chunks_dropped, 2);
+        assert_eq!(metrics.api_responses_count_per_code.len(), 1);
+        assert_eq!(
+            *metrics.api_responses_count_per_code.get(&200_u16).unwrap(),
+            2
+        );
+    }
+}