Adds open-telemetry/opentelemetry-metrics-runtime — a new contrib package
that auto-registers OTel observable instruments for PHP runtime internals:
memory usage/peak/limit, garbage collection stats (including PHP 8.3+ timing
fields), OPcache memory and hit-rate, and CPU time via getrusage().

- Disable entire package: OTEL_PHP_DISABLED_INSTRUMENTATIONS=metrics-runtime
- Disable metric groups: OTEL_PHP_DISABLED_METRICS=opcache,cpu
- OPcache and CPU metrics are skipped when underlying functions are unavailable
- Supports PHP 8.1–8.5; phan stub created via post-install-cmd on PHP 8.5+

- Use batchObserve() in CpuMetrics, GarbageCollectionMetrics and
  OpcacheMetrics so getrusage()/gc_status()/opcache_get_status() are
  called only once per collection cycle
- Handle missing ru_nvcsw/ru_nivcsw keys on OSes that omit them (macOS)
- Fix instrument units to singular form per OTel semconv: {run},
  {object}, {switch}, {hit}, {miss}, {script}, {string}
- Replace custom parseMemoryLimit() with ini_parse_quantity(); add
  symfony/polyfill-php82 to support PHP 8.1
- Register via Instrumentation SPI instead of autoload.files so metrics
  are registered after SdkAutoloader initialises Globals; remove
  _register.php
- Use separate meters per group (io.opentelemetry.contrib.php.runtime.
  {group}) to allow disabling via OTEL_CONFIG_FILE meter configurators
  or views; keep OTEL_PHP_DISABLED_METRICS as a lightweight fallback
- Depend on open-telemetry/api ^1.1 instead of sdk; move sdk to
  require-dev

- Only observe ru_nvcsw/ru_nivcsw when getrusage() actually reports them
  (absent on some OSes) instead of defaulting to 0
- Merge voluntary/involuntary context switches into the standard
  process.context_switches metric with a process.context_switch.type
  attribute, so it stays meaningful when that attribute is dropped
- Split memory usage into additive emalloc/overhead components instead of
  overlapping real/emalloc values, for the same reason
- Mark per-group metric classes as @internal
- Drop the process.runtime. prefix from PHP-specific metrics per the
  runtime metrics semantic conventions
- Pass an instrumentation version to each meter via InstalledVersions
- Add a _register.php fallback (via ServiceLoader::register()) so the
  instrumentation still registers when the tbachert/spi composer plugin
  is not allowed to run

Avoid measuring emalloc/real memory usage across observer.observe()
calls, which could allocate memory and skew the overhead calculation.
Skip reporting php.memory.limit when unlimited to keep avg/min
aggregations meaningful.

Co-authored-by: Chris Lightfoot-Wild <github-clw@wild.me.uk>

…meMetrics::register()