feat: implement size-limited JSON requests for save metrics

[pranc1ngpegasus]

During NodeX Studio's maintenance mode, the NodeX agent caches metrics in a VecDeque structure.
Once NodeX Studio becomes available again, the agent resumes sending these cached metrics.
However, there's a potential issue where the agent's cache size may exceed the maximum allowable size for JSON requests to NodeX Studio.

To address this, this PR implements a solution that creates size-limited JSON request chunks.
This approach ensures that all metrics are successfully transmitted, regardless of the cache size, by breaking down large data sets into manageable portions.

[tzskp1]

I think that intermediate types like *Str are removable.

diff --git a/agent/src/repository/metric_repository.rs b/agent/src/repository/metric_repository.rs
index 6dbc120..6eb03d3 100644
--- a/agent/src/repository/metric_repository.rs
+++ b/agent/src/repository/metric_repository.rs
@@ -34,6 +34,7 @@ pub trait MetricStoreRepository {
 }
 
 #[derive(Debug, Serialize, Clone, PartialEq)]
+#[serde(rename_all = "snake_case")]
 pub enum MetricType {
     CpuUsage,
     MemoryUsage,
diff --git a/agent/src/services/studio.rs b/agent/src/services/studio.rs
index 90c6876..a0696ca 100644
--- a/agent/src/services/studio.rs
+++ b/agent/src/services/studio.rs
@@ -413,18 +413,6 @@ impl MessageActivityRepository for Studio {
     }
 }
 
-#[derive(Serialize)]
-struct MetricStr {
-    metric_type: String,
-    value: f32,
-}
-
-#[derive(Serialize)]
-struct MetricsWithTimestampStr {
-    timestamp: chrono::DateTime<chrono::Utc>,
-    metrics: Vec<MetricStr>,
-}
-
 impl MetricStoreRepository for Studio {
     async fn save(&self, request: VecDeque<MetricsWithTimestamp>) -> anyhow::Result<()> {
         let mut metrics = request;
@@ -435,25 +423,13 @@ impl MetricStoreRepository for Studio {
             let mut current_size = 0;
 
             while let Some(m) = metrics.pop_front() {
-                let metrics_with_timestamp_str = MetricsWithTimestampStr {
-                    timestamp: m.timestamp,
-                    metrics: m
-                        .metrics
-                        .iter()
-                        .map(|metric| MetricStr {
-                            metric_type: metric.metric_type.to_string(),
-                            value: metric.value,
-                        })
-                        .collect::<Vec<MetricStr>>(),
-                };
-
-                let item_size = serde_json::to_string(&metrics_with_timestamp_str)?.len();
+                let item_size = serde_json::to_string(&m)?.len();
                 if current_size + item_size > JSON_BODY_MAX_SIZE {
                     metrics.push_front(m);
                     break;
                 }
                 current_size += item_size;
-                metrics_str.push(metrics_with_timestamp_str);
+                metrics_str.push(m);
             }
 
             let model = VerifiableCredentials::new(my_did, json!(metrics_str), chrono::Utc::now());

[tzskp1]

I think it is better to assert that item_size does not exceed JSON_BODY_MAX_SIZE like this.

                if item_size > JSON_BODY_MAX_SIZE {
                    anyhow::bail!("Invalid item size");
                }

[pranc1ngpegasus]

Thank you for your feedback.
item_size is size of JSON encoded each metrics.
current_size is total size of JSON encoded request body.
So I was implemented comparison between current_size and JSON_BODY_MAX_SIZE.

[tzskp1]

I think that your implementation has the implicit assumption: item_size does not exceed JSON_BODY_MAX_SIZE.
So I thought we should explicitly state that like below.

                if item_size > JSON_BODY_MAX_SIZE {
                    anyhow::bail!("Invalid item size");
                }
                if current_size + item_size > JSON_BODY_MAX_SIZE {
                    metrics.push_front(m);
                    break;
                }

Note: If any element has item_size > JSON_BODY_MAX_SIZE, it will loop infinitely.

[pranc1ngpegasus]

Thank you for your feedback.
I have fixed the program in the following commit:
1952571

[tzskp1]

I commented on #406 (comment) .