Outgoing HttpClient LatencyInfo enricher value has incomplete escaping

Start in src/Libraries/Microsoft.Extensions.Http.Diagnostics/Latency/Internal/HttpClientLatencyLogEnricher.cs, reading AppendServerName, AppendTags, AppendCheckpoints, and AppendMeasures. Compare their escaping behavior with the incoming enricher scheme referenced in #7729. Done means every string field escapes both commas and slashes so the eight-section and item-level layout remains parseable.

Description

The outgoing HttpClient latency enricher (AddHttpClientLatencyTelemetry)
serializes the LatencyInfo tag value as a positional string but escapes nothing,
so legal input can shift the positional layout and corrupt the value a parser
reads.

The value format is (8 sections):

version , serverName , tagNames/ , tagValues/ , checkpointNames/ , checkpointMs/ , measureNames/ , measureValues/

• Sections are joined by ,.
• List items are joined by /.

Gaps in HttpClientLatencyLogEnricher:

1. The section delimiter , is never escaped. A , in any string field
   (server name, tag name, tag value, checkpoint/measure name) shifts every
   downstream section.
2. List items do not escape /. AppendTags, AppendCheckpoints, and
   AppendMeasures append names and values raw, so a / inside an item corrupts
   the item-level split.
3. The server name field is not escaped at all (AppendServerName).

Affected code

HttpClientLatencyLogEnricher

• AppendServerName writes the server name raw.
• AppendTags / AppendCheckpoints / AppendMeasures append names and values raw, escaping neither , nor /.

Impact

Low severity. Most fields are registered/controlled names (checkpoints, measures,
tags). The one caller-controlled field is the server application name, taken from
a response header, which can legally contain , and /. Newlines are not
reachable (rejected by the server), so this is a data-integrity / parse-corruption
issue for consumers of LatencyInfo, not a log-forging issue.

Proposed fix

Escape , and / in every string field, including the server name, matching the
scheme the incoming enricher uses (/ and , replaced with _).

Notes

Discovered during API review of #7729 (AddHttpLatencyTelemetry). The incoming
enricher had the same class of defect and was fixed in that change; this issue
tracks the remaining gap in the outgoing HttpClient enricher, which currently
escapes nothing.