richat: add subscribe handshake observability

[mindrunner]

Add three metrics to diagnose 3-5s zero-byte stalls observed on new gRPC subscribers in EWR/LAX that cause clients to send RST_STREAM before any data reaches them. No behavioral fix yet — we first want one deploy's worth of data to tell us which latency actually dominates.

Metrics added:

• grpc_subscribe_filter_parse_seconds (histogram, labeled by x_subscription_id): seconds from subscribe2() entry until the SubscribeRequest is read off the wire and parsed into a Filter.
• grpc_subscribe_time_to_first_message_seconds (histogram, labeled by x_subscription_id): seconds from the filter being applied until the worker loop pushes the first data message to the client. This is the latency the client actually perceives as "time to first byte of data".
• grpc_subscribe_handshake_abandoned_total (counter, labeled by x_subscription_id): incremented when the client's request stream ends before a filter is ever set — i.e. client disconnected mid- handshake. Tells us how many stalls abandon pre-filter vs post-filter.

Plus a one-shot warn! log inside the ping task when a client has been connected for >3s without a filter set. Threshold matches the observed client timeout window.

Only the initial unset -> set transition is recorded for both histograms; subsequent filter updates (commitment change, etc.) are not part of the subscribe handshake and would skew the tails.

[fanatid]

released as richat@10.1.0 - https://github.com/lamports-dev/richat/releases/tag/richat-v10.1.0