Instrument tracing spans for block processing and import (#7816)

#7815

- removes all existing spans, so some span fields that appear in logs like `service_name` may be lost.
- instruments a few key code paths in the beacon node, starting from **root spans** named below:

* Gossip block and blobs
* `process_gossip_data_column_sidecar`
* `process_gossip_blob`
* `process_gossip_block`
* Rpc block and blobs
* `process_rpc_block`
* `process_rpc_blobs`
* `process_rpc_custody_columns`
* Rpc blocks (range and backfill)
* `process_chain_segment`
* `PendingComponents` lifecycle
* `pending_components`

To test locally:
* Run Grafana and Tempo with https://github.com/sigp/lighthouse-metrics/pull/57
* Run Lighthouse BN with `--telemetry-collector-url http://localhost:4317`

Some captured traces can be found here: https://hackmd.io/@jimmygchen/r1sLOxPPeg

Removing the old spans seem to have reduced the memory usage quite a lot - i think we were using them on long running tasks and too excessively:
<img width="910" height="495" alt="image" src="https://github.com/user-attachments/assets/5208bbe4-53b2-4ead-bc71-0b782c788669" />

book/src/help_vc.md

@@ -134,6 +134,9 @@ Options:
           Path to directory containing eth2_testnet specs. Defaults to a
           hard-coded Lighthouse testnet. Only effective if there is no existing
           database.
+      --telemetry-collector-url <URL>
+          URL of the OpenTelemetry collector to export tracing spans (e.g.,
+          http://localhost:4317). If not set, tracing export is disabled.
       --validator-registration-batch-size <INTEGER>
           Defines the number of validators per validator/register_validator
           request sent to the BN. This value can be reduced to avoid timeouts