Option to skip per-method docstring generation in build() (memory / high-concurrency)

从 build() 开始，跟踪 discovery.createMethod、schema.Schemas.prettyPrintSchema 和 prettyPrintByName，查看在构造 service 和 lazy sub-resource 期间 response-schema docstring 是在哪里生成的。复现 issue 中描述的 Sheets v4 内存配置；当一个受支持的 build-time 选项可以跳过该生成过程，并且启用后不影响正常构造时，即视为完成。

Feature request: an option to skip per-method docstring generation in build()

Problem

build() (and lazy sub-resource construction) generates a fully-expanded, recursively
pretty-printed prototype of each method's response schema and attaches it as
method.__doc__ (discovery.createMethod → schema.Schemas.prettyPrintSchema /
prettyPrintByName). For APIs with large, deeply-nested schemas this is very expensive, and it
is paid every time a service/resource is constructed.

Concrete numbers from profiling Sheets v4 (google-api-python-client==2.198.0, Python 3.13),
measured with RSS (no tracemalloc, to avoid its overhead):

• Building the service, then touching one sub-resource (service.spreadsheets(), no API
  call): ~66 MB.
• Of that, ~99.9% is the docstring schema expansion — no-oping prettyPrintSchema/
  prettyPrintByName drops it to ~1 MB. The .spreadsheets() methods themselves are ~24 KB.
• The docstrings are only useful for interactive help(); in a server they are never read.

Impact

In a concurrent server (a fresh service built per request, common with per-user credentials),
these allocations are not shared across in-flight requests. 8 concurrent Sheets requests
each build ~66 MB of docstrings simultaneously ≈ 530 MB peak, which OOM-kills a
memory-limited container. This is the concurrent-peak sibling of the long-standing
reference-cycle memory issue in #535 (whose recommended fix — build/reuse a single service — is
not always feasible when credentials differ per request).

Request

A supported way to skip docstring generation at build time, e.g.:

build("sheets", "v4", credentials=creds, generate_docstrings=False)
# or a module/env toggle

Today the only options are to monkeypatch Schemas.prettyPrintSchema/prettyPrintByName
(fragile across versions) or fork. A first-class flag would let memory-constrained / high-
concurrency deployments opt out of documentation strings they never use.

Environment

• google-api-python-client==2.198.0, Python 3.13
• Reproly: build any large-schema API (Sheets v4), touch a sub-resource, measure RSS; repeat
  concurrently to see the multiplier.

Related: #535 (memory from repeated build() / reference cycles).