CDRIVER-4010 document let option for aggregate command

Author: rcsanchez97

build/generate-opts.py

@@ -320,7 +320,8 @@ def __init__(self, items, **defaults):
         bypass_option,
         collation_option,
         server_option,
-        ('batchSize', {'type': 'int32_t', 'help': 'An ``int32`` representing number of documents requested to be returned on each call to :symbol:`mongoc_cursor_next`', 'check_set': True})
+        ('batchSize', {'type': 'int32_t', 'help': 'An ``int32`` representing number of documents requested to be returned on each call to :symbol:`mongoc_cursor_next`', 'check_set': True}),
+        ('let', {'type': 'bson_t', 'help': 'A BSON document consisting of any number of parameter names, each followed by definitions of constants in the MQL Aggregate Expression language'})
     ])),
 
     ('mongoc_find_and_modify_appended_opts_t', Struct([

src/libmongoc/doc/includes/aggregate-opts.txt

@@ -12,3 +12,4 @@
 * ``collation``: Configure textual comparisons. See :ref:`Setting Collation Order <setting_collation_order>`, and `the MongoDB Manual entry on Collation <https://docs.mongodb.com/manual/reference/collation/>`_. Collation requires MongoDB 3.2 or later, otherwise an error is returned.
 * ``serverId``: To target a specific server, include an int32 "serverId" field. Obtain the id by calling :symbol:`mongoc_client_select_server`, then :symbol:`mongoc_server_description_id` on its return value.
 * ``batchSize``: An ``int32`` representing number of documents requested to be returned on each call to :symbol:`mongoc_cursor_next`
+* ``let``: A BSON document consisting of any number of parameter names, each followed by definitions of constants in the MQL Aggregate Expression language

src/libmongoc/src/mongoc/mongoc-opts-private.h

@@ -185,6 +185,7 @@ typedef struct _mongoc_aggregate_opts_t {
    uint32_t serverId;
    int32_t batchSize;
    bool batchSize_is_set;
+   bson_t let;
    bson_t extra;
 } mongoc_aggregate_opts_t;
 

src/libmongoc/src/mongoc/mongoc-opts.c

@@ -1885,6 +1885,7 @@ _mongoc_aggregate_opts_parse (
    mongoc_aggregate_opts->serverId = 0;
    mongoc_aggregate_opts->batchSize = 0;
    mongoc_aggregate_opts->batchSize_is_set = false;
+   mongoc_aggregate_opts->let = NULL;
    bson_init (&mongoc_aggregate_opts->extra);
 
    if (!opts) {
@@ -1967,6 +1968,15 @@ _mongoc_aggregate_opts_parse (
 
          mongoc_aggregate_opts->batchSize_is_set = true;
       }
+      else if (!strcmp (bson_iter_key (&iter), "let")) {
+         if (!_mongoc_convert_bson_t (
+               client,
+               &iter,
+               &mongoc_aggregate_opts->let,
+               error)) {
+            return false;
+         }
+      }
       else {
          /* unrecognized values are copied to "extra" */
          if (!BSON_APPEND_VALUE (