feat(agent): add rust agent baseline scaffold

adk-rust/crates/awatch-agent/src/transport.rs

@@ -0,0 +1,63 @@
+use std::thread;
+use std::time::Duration;
+
+use anyhow::{Context, Result, anyhow};
+use reqwest::blocking::Client;
+
+use crate::config::AgentConfig;
+use crate::envelope::TelemetryEnvelope;
+use crate::metrics::AgentMetrics;
+use crate::spool::{FlushSummary, LocalSpool};
+
+pub fn send_envelope(config: &AgentConfig, envelope: &TelemetryEnvelope) -> Result<()> {
+    let client = Client::builder()
+        .timeout(config.request_timeout())
+        .build()
+        .context("build agent telemetry HTTP client")?;
+    client
+        .post(&config.server_url)
+        .json(envelope)
+        .send()
+        .and_then(|response| response.error_for_status())
+        .map(|_| ())
+        .map_err(|err| anyhow!("agent telemetry POST failed: {err}"))
+}
+
+pub fn flush_with_retry(
+    config: &AgentConfig,
+    spool: &LocalSpool,
+    metrics: &mut AgentMetrics,
+) -> Result<FlushSummary> {
+    let mut attempt = 0_u32;
+    loop {
+        let summary = spool.process_pending(config.retry_max_attempts, |envelope| {
+            send_envelope(config, envelope)
+        })?;
+        metrics.retry_count = metrics
+            .retry_count
+            .saturating_add(u64::try_from(summary.retried).unwrap_or(u64::MAX));
+        if summary.retried == 0 || attempt + 1 >= config.retry_max_attempts {
+            return Ok(summary);
+        }
+        let backoff = exponential_backoff(config.retry_base_backoff_ms, attempt);
+        thread::sleep(backoff);
+        attempt += 1;
+    }
+}
+
+pub fn exponential_backoff(base_ms: u64, attempt: u32) -> Duration {
+    let factor = 1_u64.checked_shl(attempt.min(10)).unwrap_or(1024);
+    Duration::from_millis(base_ms.saturating_mul(factor))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn backoff_is_exponential_and_bounded() {
+        assert_eq!(exponential_backoff(100, 0), Duration::from_millis(100));
+        assert_eq!(exponential_backoff(100, 3), Duration::from_millis(800));
+        assert_eq!(exponential_backoff(100, 99), Duration::from_millis(102400));
+    }
+}