fix(native): Fix OS metrics to report cumulative values for AVG type

[lingbin]

Fixes #26516

All 6 OS-related metrics were defined as AVG type but reported as
delta values, causing incorrect averaging and potential data loss
in Prometheus monitoring.

Changed metrics to report cumulative values since process start:

• presto_cpp.os_user_cpu_time_micros
• presto_cpp.os_system_cpu_time_micros
• presto_cpp.os_num_soft_page_faults
• presto_cpp.os_num_hard_page_faults
• presto_cpp.os_num_voluntary_context_switches
• presto_cpp.os_num_forced_context_switches

This ensures:

1. Alignment with other AVG metrics in the system (task counts,
   cache sizes, etc.)
2. Proper rate calculations in monitoring systems and no data loss
   regardless of scraping intervals

== NO RELEASE NOTE ==

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

This PR converts six OS-related metrics from reporting delta values to reporting cumulative values by eliminating subtraction of previous readings and removing obsolete state variables used for delta calculations.

Class diagram for updated PeriodicTaskManager OS metrics logic

classDiagram
class PeriodicTaskManager {
  -lastHttpClientNumConnectionsCreated_: int64_t
  +updateOperatingSystemStats()
  +addOperatingSystemStatsUpdateTask()
}

%% Removed attributes for OS metric deltas
%% lastUserCpuTimeUs_, lastSystemCpuTimeUs_, lastSoftPageFaults_, lastHardPageFaults_, lastVoluntaryContextSwitches_, lastForcedContextSwitches_ are no longer present

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Flow diagram for OS metrics reporting change (delta to cumulative)

flowchart TD
    A["Collect OS metric (e.g., user CPU time)"] --> B["Report cumulative value since process start"]
    B --> C["RECORD_METRIC_VALUE(metric, cumulative_value)"]
    %% Previously: A --> D["Subtract previous value (delta)"] --> C
    %% Now: direct cumulative reporting

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File-Level Changes

Change	Details	Files
Switch OS metrics reporting from delta to cumulative values	Removed subtraction of last recorded values when calling RECORD_METRIC_VALUE Updated RECORD_METRIC_VALUE calls to directly use current usage values for all six metrics	presto_cpp/main/PeriodicTaskManager.cpp
Remove unused state variables for tracking previous metric values	Deleted lastUserCpuTimeUs_, lastSystemCpuTimeUs_, lastSoftPageFaults_, lastHardPageFaults_, lastVoluntaryContextSwitches_, and lastForcedContextSwitches_ members	presto_cpp/main/PeriodicTaskManager.h

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#26516	Change all 6 OS-related AVG type metrics to report cumulative values instead of delta values.	✅
#26516	Ensure consistency of OS AVG type metrics with other AVG metrics in the system (i.e., all report cumulative values).	✅
#26516	Prevent data loss in Prometheus monitoring by reporting cumulative values for OS AVG type metrics.	✅

Possibly linked issues

• [native] OS Metrics(AVG Type Counters) should not report delta values #26516: The PR changes 6 OS metrics from reporting delta to cumulative values, directly addressing the issue's problem of incorrect averaging and data loss.

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[sourcery-ai]

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

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[lingbin]

@majetideepak Could you help review this PR? Thanks.

[lingbin]

@majetideepak @karteekmurthys @aditi-pandit Kindly ping. Could you please review this PR? This issue affects the accuracy of Prometheus monitoring metrics.