Add Observability terms to the Elastic glossary

[bmorelli25]

@elastic/obs-docs We need to add Observability terms to the Elastic glossary. I've included the terms and definitions that immediately came to mind below. Please feel free to edit and add your own.

Observability

• Elastic Agent (ingest)
• Observability Unify your logs, system metrics, uptime data, and application traces to provide granular insights and context into the behavior of services running in your environments.

APM

• ~APM~ ^{common term}
• Agent (APM) An open-source library, written in the same language as your service, which instruments your code and collects performance data and errors at runtime.
• Server (APM) An open-source application that receives data from APM agents and transforms them into Elasticsearch documents.
• Distributed tracing The end-to-end collection of performance data throughout your microservices architecture.
• Instrumentation Code is considered "instrumented" when it collects and reports performance data to APM.
• Real User Monitoring (RUM) Performance monitoring, metrics, and error tracking of web applications.
• Span Information about the execution of a specific code path. Spans measure from the start to the end of an activity and can have a parent/child relationship with other spans.
• Trace Defines the amount of time an application spends on a request. Traces are made up of a collection of transactions and spans that have a common root.
• Transaction A special kind of span that has additional attributes associated with it. Transactions describe an event captured by an Elastic APM agent instrumenting a service.

Logs

• Ingestion The process of collecting and sending data from various data sources to Elasticsearch.
• Integration Out-of-the-box configurations for common data sources to simplify the collection, parsing, and visualization of logs and metrics. Also known as a module.
• Module Out-of-the-box configurations for common data sources to simplify the collection, parsing, and visualization of logs and metrics. Also known as an integration.

Metrics

• ~Metric~ ^{common term}

Uptime

• Monitor A network endpoint which is monitored to track the performance and availability of applications and services.
• Uptime A metric of system reliability used to monitor the status of network endpoints via HTTP/S, TCP, and ICMP.

[bmorelli25]

I've updated the list above per the glossary guidelines discussed with @gchaps.

[bmorelli25]

@elastic/obs-docs I've added in definitions to the APM terms. Can you please add definitions to the other terms? Thanks!

Edit: Feel free to add additional terms as well!

[bmorelli25]

FYI @nehaduggal / @mukeshelastic in case you have any recommendations before these go in the docs