tricoder: add prometheus compatible metrics http endpoint

[DhanasekaranBaskar]

Summary

This PR adds a built-in text metrics exporter to tricorder, exposing all metrics via a new /prometheus-metrics/ HTTP endpoint. It also updates documentation headings to Go #-style sections.

Motivation

Tricorder currently exposes metrics via HTML, JSON, and Go RPC. This change extends tricorder’s functionality by introducing a text exposition–based metrics endpoint over HTTP, allowing external monitoring systems to scrape tricorder metrics directly without external adapters.

Key Changes

• Text exporter

  • Derives metric names from tricorder paths (e.g. /foo/bar/baz → foo_bar_baz), lowercased and sanitized.
  • Infers counter vs gauge from metric name, unit, and kind:
    • *-total metrics, network stats under /sys/netif, and selected event-style metrics are treated as counters.
    • Memory capacity and usage metrics are forced to gauges.
  • Encodes units into metric names (e.g. _seconds, _bytes, _celsius, with _total for counters).
  • Normalizes time and duration values to seconds; PrometheusNumericValue exposes the same normalized values.
  • Handles metric kinds explicitly:
    • List metrics are skipped and not exported.
    • String, bool, and time kinds use dedicated encodings (*_info, *_bool_info, *_time_seconds).
    • All other supported kinds are exported via numeric encoding.
  • Exports distributions as histograms with sorted cumulative buckets and normalized units, plus _sum and _count.

• Metric initialization

  • Uses Go-RPC-style metric representations (GoTime, GoDuration) via InitPromMetric / UpdatePromMetric, keeping the exporter independent of JSON encodings.

• HTTP endpoint

  • New /prometheus-metrics/ endpoint registered on the default HTTP mux.
  • Gzip-enabled, correct content type, and consistent trailing-slash behavior.

• Documentation

  • Updates major comment headings to Go #-style sections.
  • Notes availability of the new text metrics export.

Compatibility

This change is fully additive. Existing endpoints and APIs remain unchanged.