feat: add check_http_metrics task for Prometheus metrics assertions

[chetanyb]

Summary

• Add new check_http_metrics task for evaluating assertions against Prometheus metrics endpoints
• Supports value mode (current value) and delta mode (change over time) assertions
• Includes counter reset detection, missing metric/series handling, and label subset matching

Features

Assertion Modes:

• Value mode: Assert on current metric value (e.g., value > 100)
• Delta mode: Assert on change since baseline (e.g., counter increased by at least 1, gauge decreased)

Metric Support:

Type	Value Extracted
COUNTER	Counter value (with reset detection)
GAUGE	Gauge value
UNTYPED	Untyped value
SUMMARY	Sample sum
HISTOGRAM	Sample sum

Configurable Behaviors:

• missingMetric / missingSeries: wait, fail, or pass
• resetBehavior: fail, rebaseline, or ignore (COUNTER only)
• failOnCheckMiss: Fail immediately vs keep polling
• continueOnPass: Keep monitoring after success

Safety Features:

• Response size limit (maxResponseSize)
• Request timeout (requestTimeout)
• Non-finite value detection (NaN/Inf)
• Label subset matching must select exactly one series

Example Usage

- name: check_metrics
  task: check_http_metrics
  config:
    url: "http://localhost:9090/metrics"
    pollInterval: 10s
    assertions:
      # Check counter increased by at least 1
      - name: counter_increased
        metric: my_counter
        labels:
          env: prod
        mode: delta
        operator: gte
        value: 1

      # Check gauge decreased (negative delta)
      - name: gauge_dropped
        metric: my_gauge
        mode: delta
        operator: lte
        value: -1

      # Check current value is above threshold
      - name: value_check
        metric: my_metric
        operator: gt
        value: 100

Outputs

Output	Type	Description
passedAssertions	array	Assertion names that passed
failedAssertions	array	Assertion names that failed
values	object	Map of assertion name to latest value
deltas	object	Map of assertion name to computed delta
baselines	object	Map of assertion name to baseline value
scrapeErrors	int	HTTP/parsing error count
assertionErrors	int	Assertion evaluation error count

Files

File	Lines	Description
pkg/tasks/check_http_metrics/config.go	235	Configuration structs and validation
pkg/tasks/check_http_metrics/task.go	599	Task implementation
pkg/tasks/check_http_metrics/task_test.go	1604	37 unit tests
pkg/tasks/check_http_metrics/README.md	131	Documentation
pkg/tasks/tasks.go	+2	Task registration

Test Plan

• Unit tests pass with race detection (go test -race)
• Linter passes (golangci-lint run --new-from-rev="origin/master")
• Build succeeds (go build ./...)
• Code reviewed for correctness and style
• Manual test against real Prometheus endpoint