feat: add span duration metrics and processor for observability

[oli-bn]

• Introduced SpanToMetricProcessor to record span durations.
• Enhanced Metrics class with span and document load duration histograms.
• Updated ConfigurationDefaults to include span processor customization.
• Modified ObjectMapExporter to log span durations using the new metrics.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[MatthewVaadin]

Thank you for the contribution. Can you give some background for the use case that this is addressing?