[Internal]: OTel metrics temporality/histogram support is inconsistent across docs pages

### Description

Clarify support of explicit bucket histograms, cumulative and delta histograms.

Highlight if we have still some limitations, such as ([ref](https://www.elastic.co/docs/reference/opentelemetry/managed-inputs/managed-otlp-endpoint#limitations:~:text=Latency%20distributions%20based%20on%20histogram%20values%20have%20limited%20precision%20due%20to%20the%20fixed%20boundaries%20of%20explicit%20bucket%20histograms.)):
> Latency distributions based on histogram values have limited precision due to the fixed boundaries of explicit bucket histograms.

- Managed OTLP on Serverless
- 🟢 All supported
- Managed OTLP on ECH
- 🟢 All supported, only for 9.5+ (**only by the end of August 2026** as we're pending a change on Managed OTLP to implement a per-target-deployment version exporter usage)
- Elasticsearch OTLP endpoint (metrics) on Serverless
- 🟢 All supported
- Elasticsearch OTLP endpoint (metrics) on Elasticsearch (on-prem or ECH)
- 🟢 All supported, only for 9.5+
- EDOT GW collector configuration with OTLP Exporter to target Managed OTLP
- Depends on the target platform of Managed OTLP (see above) ([ref](https://www.elastic.co/docs/reference/opentelemetry/compatibility/limitations#managed-otlp-endpoint))
- EDOT GW collector with Elasticsearch Exporter (direct to Elasticsearch bulk API) or Managed Bulk API
- 🟡 Only delta histograms, requires `cumulativetodelta` ([ref](https://www.elastic.co/docs/reference/opentelemetry/compatibility/limitations#collector-es-exporter))

Longer term, consider a single feature-support matrix (metric feature × ingestion method × environment) so readers don't have to cross-reference multiple partially-applicable pages to work out what's supported.

### Resources

Pages involved are

* [Docs: Managed OTLP endpoint limitations](https://www.elastic.co/docs/reference/opentelemetry/managed-inputs/managed-otlp-endpoint#limitations)
* [Docs: Compatibility limitations](https://www.elastic.co/docs/reference/opentelemetry/compatibility/limitations#managed-otlp-endpoint)
* [Docs: Metric temporality](https://www.elastic.co/docs/manage-data/ingest/otlp-endpoint#metric-temporality)

### Which deployment methods does this change impact?

Elastic On-Prem and Cloud (all)

### What Elastic Stack release is this request related to?

N/A

### Serverless release

_No response_

### Collaboration model

We expect to collaborate on the first draft

### Point of contact.

**Main contact:**
* @felixbarny

**Stakeholders:**
* @lucabelluccini
* @vinaychandrasekhar
* @frconil

Start by comparing the three linked pages: Managed OTLP endpoint limitations, Compatibility limitations, and Metric temporality. Align their descriptions of explicit bucket, cumulative, and delta histogram support across the listed deployment methods, preserve the stated limitations and version notes, and finish when readers can determine supported combinations without cross-referencing contradictory guidance.

Written by the indexing model from the issue text.