feat: continuous flushing strategy for high throughput functions (#684)

This is a heavy refactor and new feature.
- Introduces FlushDecision and separates it from FlushStrategy
- Cleans up FlushControl logic and methods

It also adds the ability to flush telemetry across multiple serial
lambda invocations. This is done using the `continuous` strategy.

This is a huge win for busy functions as seen in our test fleet, where
the p99/max drops precipitously, which also causes the average to
plummet. This also helps reduce the number of cold starts encountered
during scaleup events, which further reduces latency along with costs:

![image](https://github.com/user-attachments/assets/14851e22-327d-43b0-8246-5780cfbf6ef7)

Technical implementation:
We spawn the task and collect the flush handles, then in the two
periodic strategies we check if there were any errors or unresolved
futures in the next flush cycle. If so, we switch to the `periodic`
strategy to ensure flushing completes successfully.

We don't adapt to the periodic strategy unless the last 20 invocations
occurred within the `config.flush_timeout` value, which has been
increased by default. This is a naive implementation. A better one would
be to calculate the first derivative of the invocation periodicity. If
the rate is increasing, we can adapt to the continuous strategy. If the
rate slows, we should fall back to the periodic strategy.
<img width="807" alt="image"
src="https://github.com/user-attachments/assets/d3c25419-f1da-4774-975f-0e254047b9b7"
/>

The existing implementation is cautious in that we could definitely
adapt sooner but don't.


Todo: add a feature flag for continuous flushing?

Author: astuyve

.github/workflows/rs_ci.yml

@@ -139,7 +139,7 @@ jobs:
           tool: nextest@0.9
       - uses: mozilla-actions/sccache-action@v0.0.9
       - working-directory: bottlecap
-        run: cargo nextest run --workspace
+        run: cargo nextest run --workspace --features integration_test
 
   format:
     name: Format

bottlecap/Cargo.lock

@@ -50,14 +50,14 @@ rustls-native-certs = { version = "0.8.1", optional = true }
 # method in favor of our
 # datadog_fips::reqwest_adapter::create_reqwest_client_builder. An example can
 # be found in the clippy.toml file adjacent to this Cargo.toml.
-datadog-protos = { version = "0.1.0", default-features = false, git = "https://github.com/DataDog/saluki/" }
+datadog-protos = { version = "0.1.0", default-features = false, git = "https://github.com/DataDog/saluki/", rev = "c89b58e5784b985819baf11f13f7d35876741222"}
 ddsketch-agent = { version = "0.1.0", default-features = false, git = "https://github.com/DataDog/saluki/" }
 ddcommon = { git = "https://github.com/DataDog/libdatadog", rev = "d6a2da32c6b92d6865a7e7987c8a1df2203fb1ae" }
 datadog-trace-protobuf = { git = "https://github.com/DataDog/libdatadog", rev = "d6a2da32c6b92d6865a7e7987c8a1df2203fb1ae"  }
 datadog-trace-utils = { git = "https://github.com/DataDog/libdatadog", rev = "d6a2da32c6b92d6865a7e7987c8a1df2203fb1ae" , features = ["compression"] }
 datadog-trace-normalization = { git = "https://github.com/DataDog/libdatadog",  rev = "d6a2da32c6b92d6865a7e7987c8a1df2203fb1ae" }
 datadog-trace-obfuscation = { git = "https://github.com/DataDog/libdatadog", rev = "d6a2da32c6b92d6865a7e7987c8a1df2203fb1ae"  }
-dogstatsd = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a", default-features = false }
+dogstatsd = { git = "https://github.com/DataDog/serverless-components", rev = "b1583da00e202985057cf6d48f05e1cac77ab910", default-features = false }
 datadog-trace-agent = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a" }
 datadog-fips = { git = "https://github.com/DataDog/serverless-components", rev = "3f3347790d4c7f3393589ef7efb7ccf22405a89a", default-features = false }
 axum = { version = "0.8.4", default-features = false, features = ["default"] }
@@ -97,3 +97,4 @@ fips = [
     "rustls/fips",
     "rustls-native-certs",
 ]
+integration_test = ["reqwest/rustls-tls", "dogstatsd/default", "datadog-fips/default" ]

bottlecap/Cargo.toml

@@ -4226,6 +4226,214 @@ third_party_libraries:
 
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@@ -6857,7 +7065,7 @@ third_party_libraries:
             same "printed page" as the copyright notice for easier
             identification within third-party archives.
 
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+         Copyright 2025 Datadog, Inc.
 
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