Add OpenTracing for database activity. (#10113)

This adds quite a lot of OpenTracing decoration for database activity. Specifically it adds tracing at four different levels:

 * emit a span for each "interaction" - ie, the top level database function that we tend to call "transaction", but isn't really, because it can end up as multiple transactions.
 * emit a span while we hold a database connection open
 * emit a span for each database transaction - actual actual transaction.
 * emit a span for each database query.

I'm aware this might be quite a lot of overhead, but even just running it on a local Synapse it looks really interesting, and I hope the overhead can be offset just by turning down the sampling frequency and finding other ways of tracing requests of interest (eg, the `force_tracing_for_users` setting).

1 files changed, 6 insertions, 0 deletions

diff --git a/synapse/logging/opentracing.py b/synapse/logging/opentracing.py
index f64845b80c..68f0c00151 100644
--- a/synapse/logging/opentracing.py
+++ b/synapse/logging/opentracing.py
@@ -271,6 +271,12 @@ class SynapseTags:
     # HTTP request tag (used to distinguish full vs incremental syncs, etc)
     REQUEST_TAG = "request_tag"
 
+    # Text description of a database transaction
+    DB_TXN_DESC = "db.txn_desc"
+
+    # Uniqueish ID of a database transaction
+    DB_TXN_ID = "db.txn_id"
+
 
 # Block everything by default
 # A regex which matches the server_names to expose traces for.