[YoussefElshemi]

Closes #1854

Adds built-in tracing and metrics using System.Diagnostics ActivitySource and Meter, both named Renci.SshNet. OpenTelemetry (or any other listener) picks them up with:

builder.Services.AddOpenTelemetry()
    .WithTracing(t => t.AddSource("Renci.SshNet"))
    .WithMetrics(m => m.AddMeter("Renci.SshNet"));

When nothing is listening, SshNetTelemetry.IsEnabled is checked first, so no spans, tags or timestamps are created.

What is instrumented

Each operation produces a Client span named {protocol} {operation} (e.g. sftp UploadFile) and a ssh.client.operation.duration histogram measurement (seconds).

• Connect for SshClient, SftpClient and NetConfClient (ssh / sftp / netconf), sync and async
• SFTP: UploadFile, DownloadFile, ListDirectory, ChangeDirectory, ChangePermissions, CreateDirectory, DeleteDirectory, DeleteFile, Delete, RenameFile, SymbolicLink, Get, Exists, GetStatus, GetAttributes, SetAttributes — sync, async and Begin/End where they exist
• SshCommand execution (ssh Execute)

Not instrumented:

• SCP (ScpClient is unchanged, as discussed in the issue; its Connect still goes through BaseClient and shows up as ssh Connect)
• Stream-based APIs (Open*, ReadAll*, WriteAll*, AppendAll*), since a span would only cover opening the handle, not the I/O
• Command text, as it may contain secrets

Attributes

Attribute	Source	Span	Metric
network.protocol.name	semconv	✓	✓
server.address, server.port	semconv	✓	✓
error.type	semconv	✓	✓
file.path	semconv	✓	(high cardinality)
ssh.operation.name	custom	✓	✓
sftp.transfer.bytes	custom	✓
ssh.command.exit_code	custom	✓

There is no SSH/SFTP semantic convention yet, so the custom names should be treated as experimental. Failed operations set span status to Error and record error.type (exception type name). Exists returning false is not an error.

Notes for review

• SshCommand.ExecuteAsync isn't an async method, so it restores Activity.Current after starting the span; otherwise the caller would continue inside the command's span.
• The command span is ended before the TaskCompletionSource is completed, so it has ended by the time awaiters resume. It is also ended if the command is disposed mid-run.
• Delete and ChangePermissions call Get internally, so their spans have a child Get span.
• New dependency: System.Diagnostics.DiagnosticSource 8.0.1 on net462/netstandard2.0 only (newer TFMs have it in-box).
• Public Connect/ConnectAsync in BaseClient now wrap private ConnectCore/ConnectCoreAsync; no public API changes.

Testing

• Release build clean on all targets
• Unit tests pass (net10.0)
• New TelemetryTests integration tests (spans, parenting, attributes, durations, errors, command timeout/exit code) pass on net48, net8.0, net9.0 and net10.0
• Existing SFTP / SshClient / connectivity integration tests pass (net10.0)
• Also checked the output manually against the .NET Aspire dashboard

This change was developed with AI assistance. I have reviewed and tested all of it.