feat(config): add pull metric reader support to declarative config

[MikeGoldsmith]

Description

Adds pull metric reader support to declarative file configuration, including Prometheus metric reader via the prometheus_development config field.

Previously, _create_metric_reader() raised ConfigurationError for any pull reader config. Now it dispatches to new factory functions that handle both the built-in Prometheus reader and plugin-based pull readers.

How it works

Prometheus (prometheus_development):

• Dynamically imports PrometheusMetricReader and start_http_server from opentelemetry-exporter-prometheus (same try/except pattern as OTLP exporters)
• Maps config fields: without_target_info_development → disable_target_info, host/port → start_http_server()
• Warns for unsupported fields: without_scope_info, with_resource_constant_labels

Plugin pull readers:

• Unknown exporter names in additional_properties are loaded via load_entry_point("opentelemetry_pull_metric_exporter", name)(**(config or {}))

Not yet supported (warnings logged):

• producers — SDK doesn't have MetricProducer support yet (feat(config): add pull metric reader support to declarative config #5074)
• cardinality_limits — deferred

Design note

Pull "exporters" like Prometheus are combined reader+exporter objects — PrometheusMetricReader IS a MetricReader. The _create_pull_metric_exporter() function returns a MetricReader, matching the config schema's terminology while reflecting the SDK's class hierarchy.

Example config

meter_provider:
  readers:
    - pull:
        exporter:
          prometheus_development:
            host: "0.0.0.0"
            port: 9090
            without_target_info_development: true

Tests

8 new tests covering: prometheus creation with config, defaults, missing package, no exporter, plugin loading, plugin not found, producers warning, cardinality_limits warning.

Closes #5073

[herin049]

No need to add this into this PR, but it might be nice to define an exception specifically for dependency errors, e.g.

class MissingDependencyError(ConfigurationError):
    """Raised when an optional dependency is not installed."""

    def __init__(
        self,
        package: str,
        feature: str | None = None,
        install_name: str | None = None,
        extras: str | None = None,
    ) -> None:
        self.package = package
        self.feature = feature
        self.install_name = install_name or package
        self.extras = extras

        if extras:
            install_cmd = f"pip install '{self.install_name}[{extras}]'"
        else:
            install_cmd = f"pip install {self.install_name}"

        if feature:
            message = (
                f"{feature} requires '{package}'. "
                f"Install it with: {install_cmd}"
            )
        else:
            message = (
                f"'{package}' is required but not installed. "
                f"Install it with: {install_cmd}"
            )

        super().__init__(message)

[herin049]

Looks like lint is failing.