feat(native): Add counter to track http client connection reuse

[kewang1024]

== NO RELEASE NOTE ==

[sourcery-ai]

Reviewer's Guide

Adds HTTP client metrics to distinguish first-use versus reused connections using transaction sequence numbers, and registers the corresponding counters in the metrics system.

Sequence diagram for HTTP client connection reuse metrics

sequenceDiagram
    participant Client
    participant HttpClient
    participant HTTPTransaction
    participant ResponseHandler
    participant MetricsSystem

    Client->>HttpClient: sendRequest()
    HttpClient->>HTTPTransaction: createOrReuseTransaction()
    HTTPTransaction->>ResponseHandler: setTransaction(this)
    ResponseHandler->>HTTPTransaction: getSequenceNumber()
    HTTPTransaction-->>ResponseHandler: seqNo

    alt seqNo == 0 (first use)
        ResponseHandler->>MetricsSystem: RECORD_METRIC_VALUE(kCounterHttpClientConnectionFirstUse)
    else seqNo > 0 (reused connection)
        ResponseHandler->>MetricsSystem: RECORD_METRIC_VALUE(kCounterHttpClientConnectionReuse)
    end

    ResponseHandler-->>HTTPTransaction: transactionHandlerReady()
    HTTPTransaction-->>HttpClient: transactionReady()
    HttpClient-->>Client: responseProcessed

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Flow diagram for HTTP client connection reuse counters definition and usage

flowchart TD
    A[Headers in Counters_h<br/>kCounterHttpClientConnectionFirstUse<br/>kCounterHttpClientConnectionReuse] --> B[Metrics registration in Counters_cpp<br/>registerPrestoMetrics<br/>DEFINE_METRIC for both counters]
    B --> C[Metrics system<br/>stores SUM counters]
    C --> D[HttpClient ResponseHandler_setTransaction<br/>txn_.getSequenceNumber]
    D -->|seqNo == 0| E[RECORD_METRIC_VALUE<br/>kCounterHttpClientConnectionFirstUse]
    D -->|seqNo > 0| F[RECORD_METRIC_VALUE<br/>kCounterHttpClientConnectionReuse]

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File-Level Changes

Change	Details	Files
Instrument HttpClient to count first-use vs reused connections based on transaction sequence number.	In ResponseHandler::setTransaction, fetch the HTTPTransaction sequence number when the transaction is set. Record kCounterHttpClientConnectionReuse when sequence number is greater than zero and log a VLOG(2) message indicating reuse. Record kCounterHttpClientConnectionFirstUse when sequence number is zero, representing the first request on a connection. Document how sequence number maps to connection reuse semantics in comments.	presto-native-execution/presto_cpp/main/http/HttpClient.cpp
Define and register new counters for HTTP client connection first use and reuse.	Add two new counter names for HTTP client connection_first_use and connection_reuse in Counters.h with brief documentation comments. Register the new counters in registerPrestoMetrics using DEFINE_METRIC with StatType::SUM so they are exposed to the metrics subsystem alongside existing HTTP client metrics.	presto-native-execution/presto_cpp/main/common/Counters.h presto-native-execution/presto_cpp/main/common/Counters.cpp

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Hey there - I've reviewed your changes and they look great!

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